i don't know about a store but for my friend i bought lots of coloured dummies (pacifiers) and tied the invitation to them nice if you know sex of the baby

If you're good with the computer, you can make your own and buy specialty paper from an office supply store like Staples or Office Depot. They may even create them for you if you directed them how you wanted it to look.

Or if you want to try doing it yourself, you can even do a search and find pre-made templates online that you can customize and print out.

You Can Use Microsoft Publisher to design one, print it out, and take it to be copied at, say, staples

I have a party planning business, i always design my own and call a printing shop, they will print them in color for very cheap.. if you have them design it, it costs more. They also will have envelopes for them, so you can get them all from one place. I used my daughters picture for her 1 year invitations they turned out really cute! Good luck!