Hi M.,
If you have created the announcements cards yourself, it depends on the size you have created.
I do card design myself and sell most of my stuff on eBay and online. For DYI printing, the best resource is Usually WalMart. The cheapest and easiest way is to create a 3.5x6 image. Once you're done, size it to 4x6 with the excess 1/2 inch blank. send it to Walmart or another printing place. Once they are printed, you can cut off the excess 1/2 inch, and they easily place inside regular envelopes which you can get good ones of for $2-$3 for 100 qty.
If you want to go a little more fancy, you can use Mpix. They are great with templates, or DIY. I love the 4x8 sizes, especially because they come with envelopes.