Buy JR Hokkaido tickets in Haneda airport

Can I purchase train tickets to travel from New Chitose Airport (Rapid Airport 165 for OTARU) to Minami-Chitose station, and connection tickets from Minami-Chitose station to Tomamu station (Ltd. Exp. Super Tokachi 5) ahead of time when I land in Haneda Airport (July 12 2019) - as I have several hours of layover time in Haneda before my connecting flight to New Chitose Airport, and my landing time in New Chitose Airport is very close to the departure time of the trains so I am worried that I can't make it if I don't buy ahead of time. Please help, thank you.

Go here.
https://www.jreast.co.jp/e/customer_support/service_center_haneda.html

But in that trains not be full, you can buy it at CTS.
http://www.new-chitose-airport.jp/en/access/jr/
http://www2.jrhokkaido.co.jp/global/english/travel/info.html
https://goo.gl/maps/ZWZkmS6rbR2qZicP7

Yes. You can buy/reserve JR tickets at any JR ticket office up to a month in advance. So you can get your Hokkaido tickets at Haneda.

thank you so much for the valuable advices - I will go ahead and get the JR Hokkaido tickets when I land in Tokyo/Haneda, at the Haneda airport JR ticket/services window. I assume I can also get the return tickets (from Tomamu to New Chitose Airport) when I buy tickets in Haneda airport - sinceI I read that Tomamu station is unmanned?

Yes, you can do the return ticket from Tomaru Station at Haneda. There would most likely be a ticket vending machine onsite, but the helpful google says you can get tickets nearby. Looking at a map, the expression "nearby" might not be very accurate. Haneda looks like a good option in this case.

The station is unstaffed.
There is a JR reservation office in the hotel near the station.

perfect - Haneda airport it is then - thanks!