I make them! Have been every year now too for like, 20 years or so! I buy the blank ones at a craft store and then I pick a theme and make tonnes! You should use one of your beautiful photos Amy, print them out and then glued on a card - easy peasy, beautiful and unique!
Yes, they are prefolded and I can get them in several stores around me - Michael's always has a good selection. I have gotten a photo printed and then stuck it on the card or sometimes I run them though my computer. They always have some sort of embellishment on them too. Here's a couple of mine:
I used to make them with the pre made cards. But, now I just get the photo cards. I like zazzle or shutterfly. Zazzle has fantastic designs but are a bit spendy. But, you can't beat shutterfly for prices!
I am SUPER cheap and have gone to great lengths to get cheap cards.
I've formatted our cards onto 4x6's and ordered them at Walgreens as regular photos when they are on sale.
I've also ordered them through vistaprint as 4x5 flyers/postcards and got 300 for like $5.00 one year.
I think that was most clever because I sent it as a postcard and they were only .17 cents in postage as well. I was able to put a picture on the front and a little message on the back plus room for addressing. However, it ended up being kind of small, so next time I would do a larger flyer.
Before that, I always made them. I also have a stash of photo cards I bought after Christmas on sale to put a picture in (just have to make sure they don't say "Merry Christmas 2003" on them).