Staples and other big box office stores do have larger quantity boxes. And the quality isn't bad. But $4 for box of 250? I wish but No Way! I just priced some the other day there and I think it was about $9 for a box of 100.
Currently, I buy mine at WalMart just for the convenience. They have a box of 60 for about $4.50. The quality isn't quite as nice as other places, but I figure that as long as my cards get to where they are going, it doesn't matter if the envelope is super nice. It only gets ripped open and tossed anyway.
If you need to buy a lot of envelopes though, online would probably be your best bet.