I'm in the loooong process of using up all the sheets of DCWV paper I have from the big Winter, Spring, Summer, and Fall stacks they do every year. I bought the stacks four years ago and still have plenty of sheets left, although I finally finished Winter on last year's Christmas cards.
I do the envies white side out, but if you Google Easy Peasy Three Minute Envelope, you'll see that she makes her envelopes color side out. I use her tutorial and don't fool with any of the special envelope makers out there. If my cards are bulky, I make the envelope slightly larger and slip in a piece of bubble wrap. So far they've arrived in good condition.
I use the single-sided patterned paper which I don't like very much.
Or you can use white paper to make the envelope itself, and line it with leftover patterned paper to strengthen it. Just cut a house-shape a little smaller than the size of the opened envelope, leaving a white edge for your sealing adhesive, and score the PP where the flap folds. Slot the PP into the envelope, coloured side up, and stick to the flap only. That way it won't buckle when you close the envelope.
My mailman told me I cannot mail things that are all patterned on the outside of the envelope. I tried anyway... and mailed something in a patterned paper envelope to my sister. She never got it and it was not returned to me. This experience led me to believe that I should do what the mailman said! Also... note the square envelopes cost more to mail than rectangular and that even rectangular ones of irregular size will need more postage. If you could make a template the size of a regular card envelope, you could follow it and put the white side of the paper on the outside and the fun side towards the inside.
I too have wondered what to do about envelopes. I just ordered some from Amazon, but of course now I find that I need to order some more boxes in various sizes since just one size envelope does not fit all of the different size cards I make. Best of luck in finding envelopes.
If you like the look of the pp on the outside, you could make that kind of envelope for hand-delivered cards or ones that go on top of a present, or something like that.
As for square cards, there's no law that says they have to go in square envelopes. I put my 5x5 cards in a 5.5x7 envelope, and they're good to go. What you save in postage, you can spend on your next shopping trip!