Hi Chrissy!
I can usually find invitation envelopes (they are just slightly larger than 4.25 x 5.50) in office supply stores. Sometimes I can find them at Walmart. It's been awhile so I have no idea what the price is but I think the box has 100 envelopes. I personally like the brightness of the white envelopes from Office Max.
I have purchased some square envelopes (usually vellum) at Michaeal's or A.C. Moore (either on sale or with a coupon). I try to have 10-20 each to accommodate 5x5 and 6x6 square cards.
And, esp. when the card will be given in person, I use the clear envelopes sold by SU.
I know Marco's Papers has wonderful service and decent prices. They will have a booth at the Heirlloom Show in Rhinebeck (Sept. 20-21, 200
.
Hopefully others will chime in with vendors for bulk buys and odd sizes. This info is always good to know!
ETA: I also have a Coluzzle template for envelopes but haven't used it yet - I needed the scoring tool (which I just git from my BFF/TAC demo).