I purchase deli cups (32 oz) with lids from a wholesaler called Bruce Edmeades. I was buying from a local Loblaws.... but they directed me straight to the wholesaler, as it was kind of a pain in the a$$ for them to wrap up a sleeve of delis for me every week. Have a look in the yellow pages, and you may find a wholesaler of some sort in your area. They don't sell to you at the true wholesale price that they would to the supermarkets, but it is still a heck of a lot cheaper than buying from the supermarkets.
To make the lids "fly friendly," I cut a circle out of the lid with a hobby knife (it doesn't have to be perfect.. a square hole would work too), and I glue a circle cut out of a coffee filter onto it to make it flyproof using childrens glue. This keeps out the wild-type flies, and keeps the mutants and their larvae in. It has been working well for me so far... I just thought I'd share.
Good luck
Double J