Two options:
1 - a place that does printing, like 4-color printing. You can get stuff printed on the front and back, on card stock, etc.
2 - a photo printer like WHCC. Just make a ton of 4x6 prints. The print quality will likely be higher, but it will be on photo paper not on card stock. Also probably won't have printing on the back.

Of course, some places do both. Just a thought.