Did you know there is a little resource that you probably haven't tapped yet? It's called your local card shop. Yup, your card shop.
I used to work at a little boutique that sold cards, stationary, gifts and did custom printing. One interesting thing that I learned was that after a card season is over, you send back all the cards you don't sell, but you do not send back the envelopes. So that leaves the store with sometimes hundreds of random envelopes.
Which can become, that's right, your free envelopes! Yours for the taking!
Kids love envelopes. Seriously, nothing is more fun then opening one, or tucking something special inside. It's a great motivational tool.
I've been giving my 5th graders their post assessments for science in an envelope. They actually can't wait to open them to see what task they need to accomplish and then "send" it back to me. Really, kids actually wanting to take post assessments. It's a miracle!
They also come in handy when making those various holiday cards to "send" home to family. Kids will be so proud when they can present their homemade card in an "real card" envelope.
So call up your local card shop. I would call independently owned shops first, before chain stores like Hallmark or Papyrus, you'll probably have better luck at those. Tell them you're a teacher (or homeschool mom), ask them if they save their extra envelopes, and would they be willing to give you some. Some shops may not save them, but ask them if they would be willing to save some for you to come pick up. Call right before major card selling holidays (i.e. Valentine's Day, Mother's Day, etc.) and you will probably have a few hundred envelopes you can pick up within a few days.
Be sure to thank them by frequenting their shop and sending business their way!