Hi Rob,
Where are you located? If in the UK, you can mail-order via the internet through Silverprint in London (see APUG ad as they're an advertiser here), Retro Photographic, Jessops etc. You can even buy through Amazon.
If that doesn't help, Google is your friend.
As for E-Bay, unless you want unique film like Kodak Infrared, I wouldn't bother. Film is often good for months or years after its best-by date, but that depends on storage. As the others have said, it's too much of a risk, you can't see how it's been stored and you'll probably waste your money.
OTOH, shop rummage bins can be a useful way of buying out of date film, and it's nearly always been refrigerated. And rummaging is fun.
What do i consider the limit in shooting 120? Well, if and when it's no longer available, i'll have no choice but to stop using it. But i can't see that happening for a while. 35mm was supposed to have killed off roll film in the 1960's.
When the cost becomes prohibitive - maybe i'll stop, but i didn't pay all that money for my gear just to gather dust.