My LFS uses 4oz specimen cups to send frags home. Needless to say because of that I have a decent stock. It's a great concept. AsI start fragging some of my softies I will use them. If you search for 4oz specimen cups on amazon there are some good deals.
I buy urine cups on Amazon in the big packages. To hold the frag still during shipping I use a pool noodle chunk and poke a hole in it. That way the plug hangs upside down and fits snug. No movement at all or leakage!
You looking for cups to ship in or to transport to frag swaps and stuff? If it's the 2nd, ifragit.com has some small cups I use for bringing stuff home from frag swaps/trade shows.
I get them from my lfs whenever I pick up a frag and save them. Amazon has many choices if you're looking for quantity. I recently received a WD frag and the person who sold it to me glued the frag under the cap so it was upside down, made it from CA to NY with no issues.