Here's a free popup card created using PopUp Card Studio (it's in a .zip file). It's not all fancy, but just a base card for you to use. You can cut paper to fit the pieces and add your own graphics. If you use the file I'd love to see it all decorated.
There is also a small envelope included in the file. If you have trouble with this file, let me know because I didn't test it yet.
If you'd like to try Popup Card Studio there is a free demo. You can purchase it too. I'm an affiliate, so if you purchase you will be helping me get a small percentage of the sale.