Hi, and welcome to SCS. I'm sorry no one has responded to your post so far.
It may be because I think many who make cards don't buy the cards themselves, but make them by cutting 8-1/2" x 11-1/2" cardstock in half at the 5-1/2" mark, and then folding to make an A2 size card, ie 4-1/4 x 5-1/2.
The envelopes to fit an A2 size card are usually called "invitation" size and can be found at any number of places, online, and also at office supply stores like Office Depot and Staples. Depending on the quality you want, and the number of envelopes you buy, the prices vary greatly.
If you want to go the Stampin' Up route, you can easily find a demonstrator in your area, and order supplies from SU. Just google Stampin' Up.
Hope this helps.