I had run out of enveopes and borrowed some from my brother who always buys more than enough. His current favorites at that time were Redi Seal double flap envelopes available from OfficeMax.
These enveoples are regular business size (#10), white, the self-stick kind that you don't need to lick (yuck) or wet with a sponge. I was used to peel-and-seal ones, but these are a little dfferent: they have two flaps instead of one, both covered with adhesive. In order to seal the envelope, you lift the lower flap, put the upper one on it, and press them together. While the lower flap is down, the envelope will not close by itself or if you accidently press on the flap - very smart design. I really like how it works.
The envelopes are sold by boxes of 500. I am not sure I need so many at the moment, but I might stock up on them anyway. I occasionally need to mail out quite a lot of business-related materials, and these envelopes are far more convenient than regular ones.