I buy mine at the local paper company. I know not everyone has one locally but I know places like Xpedx are all over the country. (But don't go looking for their stores on their site--I have never found all the stores on there and I can't figure out why!) My local company is Lewis Paper and I pay about $12-$14 for 250 envelopes.
Debra (above) seems to have found a great deal but I couldn't find anything that inexpensive on clearbags site. The best I found was Mohawk at $61 per thousand envelopes. Maybe she has a link to the envelopes she found?
Quiltzi...High Plains Member of PunchkateerzI'll stand with arms high and heart abandoned,
in awe of the One who gave it all.
I'll stand, my soul, Lord, to You surrendered,
all I am is Yours.