Where to meet seiyuus in Japan?

I'm really crazy about seiyuus like Hikaru Midorikawa and Koyasu Takehito and would love, love, love to meet them in person. Where is a good place to meet seiyuus and does anyone know where they typically hang out in Tokyo?

It's my first time to find a word "seiyuu" (voice actor / actress) used in English.

I suppose you had better cool down and check whether a seiyuu whom you hope to meet or his/her agency is offering such an opportunity.
When you see a seiyuu on a street, he/she may well be in his/her private time, for example seeking for a present for his/her family member(s).

Some seiyuus, like a voice actress Mizuki Nana, are publicizing his/her profile as an entertainer and acting also as a singer, a talent or so with his/her face being open to the public.
A voice made by such a seiyu can make listeners form an image based on his/her features including visual ones.

However, I guess there are many seiyuus who are not letting his/her personal info (other than voices) known widely to listeners.
For example, when a seiyuu is making a voice of an anime character, some listeners form an image of the character according to the voice, not knowing other features of the seiyuu.
An anime seiyuu may think of hiding his/her non-voice features, for them not to intervene in images which listeners have already formed.

seiyuus like Hikaru Midorikawa and Koyasu Takehito

For your information: The name Hikaru Midorikawa is Midorikawa Hikaru in Japanese, while the names Koyasu Takehito and Mizuki Nana have the same word order in Japanese.

I don't think it is possible for you to meet seiyuu in public/ be able to meet them where they 'hang out.' Most likely, seiyuu will want their own private time and not interact with fans outside of events.

You could try handshake events or autograph events, where you get a ticket from a CD or something new which has been released by the seiyuu. This is where the 'ticket' is entered into a ballot system and if your card was drawn, then you are allowed to go to a handshake or autograph event where you meet the seiyuu.
No guarantees though, as it is very hard to get a ticket into one of these events.

Where is a good place to meet seiyuus and does anyone know where they typically hang out in Tokyo?

Seiyus, like other actors and celebrities, are not really available to hang out with fans. However you may have the opportunity to meet them at publicity events and such. Check out their fan pages or places like the Anime Center in Akihabara where different Seiyus sometimes make appearances.

look into conventions and akiba instore events and the like, they usually have such people