Mostly ebay
I find that most traditional B&W films will still function well years after thier 'expiry' date, so I harvest my roll film from out of date/short dated film sales on E-bay.
The last one involved a lot including a mix of about 25 rolls C-41, about 80 rolls E-6 and about 60 rolls of B&W for about $80, and I was able to arrange free local pickup to boot. Now, as to geting food into the freezer...
I am using the E-6 rolls to educate myself about studio lighting. I self process it and have the results availiable while the rec room is still set up for whatever I was working on shooting.