Tokyo Subway Ticket & Public transport

a) May i know where can i buy this Tokyo Subway Ticket in Haneda Airport, information counter or which floor office?
b) As my flight reach Haneda airport will be night time about 11pm, is there any public transport which i can access & reach Dome Hotel?

You can buy at Tourist Information Center on the second floor of International terminal, arrival lobby which open 5:00 - 25:00 7days.
You will need to present your passport when you buy it.

For Tokyo Dome Hotel, I would suggest, go to Hamamatsu-cho station by Tokyo monorail(from Haneda), then switch to Yamanote-line(green train) departs from platform No.2, then switch to Sobu-line(yellow train) departs from platforn No.5 for Suido-bashi station.
You can have access from Suido-bashi both east and west exit to the hotel. The hotel recommends you to get through east exit.
You better not to use Tokyo subway ticket to the hotel. It will be cheaper but it takes longer to get to the hotel at mid night.

where can I buy this Tokyo Subway Ticket in Haneda Airport,

A tourist holding his/her foreign passport can purchase a Tokyo Subway Ticket
at the ticket booth (05:30 - 01:00) of Tourist Information Center in Arrival Lobby on the 2nd floor of the HND International Passenger Terminal.
http://www.haneda-airport.jp/inter/en/premises/service/rental_car.html...
http://www.haneda-airport.jp/inter/en/map/floormap/map.html?floorId=MA...

Or, you can purchase one later at a BIC CAMERA store in Tokyo which handles Tokyo Subway Tickets for foreign tourists.
http://www.tokyometro.jp/en/ticket/value/travel/index.html#anc03

is there any public transport which I can access & reach Dome Hotel?

You had better plan to stay at a hotel located in a shorter distance for that night.

According to the current timetables,
in order to reach Tokyo Dome Hotel before 04:00 mainly by railroad,
you need to leave the HND International Passenger Terminal on the 23:45 monorail train at latest.
Because your plane is scheduled to arrive around 23:00, if everything goes smoothly you can do it, but you may miss the last connection if your flight or immigration procedure is delayed much.

Logically,
if you catch the 00:00 limousine bus for Shinjuku, which is scheduled to arrive to Shinjuku Station West Exit terminal at 00:28,
you can take the last 00:40 eastbound Local train on Chūō Line (departing at Track 13) from Shinjuku Station (of JR), which is scheduled to arrive to Suidōbashi Station at 00:52.
However, practically you cannot book this bus in advance, and you need to hurry in Shinjuku especially if you have much luggage or the bus is delayed, and you may miss the last train if you lose your way.
The transportation cost on this route is 2170 yen per adult passenger.

A taxi fare is estimated at 8300 yen per car if you take a taxi all the way from the HND International Passenger Terminal to Tokyo Dome Hotel during 22:00 - 05:00, which may surpass your accommodation cost for one night.

So, as long as you take the mentioned flight, I recommend you would check the cancelation policy of that hotel and examine possibility of staying in a nearby hotel.

- Royal Park Hotel The Haneda
http://www.rph-the.co.jp/haneda/en/
: This hotel is adjacent to the international passenger terminal.
- Hotel JAL City Haneda Tokyo
http://www.haneda.jalcity.co.jp/eng/map_timetable.html
: Shuttle buses to this hotel are available until 00:40 at the group bus spot in the international passenger terminal.

How about if i take a taxi (which can accommodate 6 persons & 7-8 luggage) from Haneda airport to Tokyo Dome Hotel?

A taxi in Tokyo carrys 5 passengers include the driver, you need to hire 2 taxis if you are a group of six. I think taxi fare costs you between 8,500 - 9,000 Yen per taxi.
You can catch taxis from Hamamatsu-cho station to reduce the fare cost for Tokyo Dome Hotel.

How about if I take a taxi (which can accommodate 6 persons & 7-8 luggage) from Haneda airport to Tokyo Dome Hotel?

Then you need two taxi cars.
A taxi car in a typical size can carry up to 4 adult passengers; even when only one of the passengers is 12-or-more years old, he/she can accompany up to 4 children under 12.

When the taxis run by only toll-free sections, your taxi rides may last for about 50 minutes and the taxi fares are expected to be about 2800 yen per passenger (#1).
#1: 2 taxi cars X 8300 yen per taxi car / 6 passengers = 2766.66... yen / passenger.

When the taxis run mainly by expressway, you may reach the hotel in 30 minutes or so, but you are expected to pay additionally about 900 yen per taxi car (i.e. about 300 yen per passenger) as expressway tolls.

----

I noticed a connection to Tokyo Dome Hotel which is later than the ones mentioned in my previous reply.

[Route by Monorail, JR and taxi via Kanda Station]

If you catch the last 00:10 monorail train to (Monorail) Hamamatsucho Station, you can take a JR train on Keihin-Tohoku Line to Kanda Station and a taxi to the hotel.
The one-way transportation cost on this route is estimated at about 1050 yen per passenger.

Monorail Entrance Gate:
Haneda Airport International Terminal Station {00:10}
| [Tokyo Monorail, Express: 490 yen]
{00:25} (Monorail) Hamamatsucho Station
: --Escalator to go down-- Monorail Central Exit Gate
https://www.jreast.co.jp/e/stations/e1248.html
| Escalator to go down;
| Escalator to go up;
South Entrance Gate:
Hamamatsucho Station: Track 1 {00:34 / 00:51}
| [JR Keihin-Tohoku Line (northbound), Local: 160 yen]
{00:42 / 00:59} Track 4: Kanda Station
: --Escalator to go down-- Gate: East Exit
https://www.jreast.co.jp/e/stations/e538.html
| [Taxi: about 400 yen / passenger (#2): 10 minutes or so]
Tokyo Dome Hotel.
#2: 2 taxi cars X 1180 yen per taxi car / 6 passengers = 393.33... yen / passenger.