Depending on the size and formality of your RD, you may not even need to do formal invitations. For our RD, we sent a Facebook message to our WP members and their SOs. We just called grandparents and invited them, and our immediate families already knew the details. I just kept track of responses on an Excel spreadsheet. Total cost: $0. However, if you are having a larger or more formal rehearsal dinner, and you want to send out actual invitations, making your own or using a site like VistaPrint is probably your best bet.
For my wedding invites (and all other stationary I used for wedding events), I had a graphic designer on Etsy make a custom design for us, then I just printed them myself. You can also print very inexpensively through Vistaprint. I liked doing it this way because I got a unique design that wasn't cookie-cutter; however, it saved me a ton!
Here's an example of a shop that does custom or pre-done designs on Etsy. I bet that any of the stuff on there would work for a rehearsal dinner invite, too:
Re: Rehearsal Dinner Invites
Or, use these. Pretty much as inexpensive as it gets.
http://www.amazon.com/Rehearsal-Dinner-Wedding-Anniversary-Invitations/dp/B00A7PWWM2/ref=sr_1_7?ie=UTF8&qid=1364312037&sr=8-7&keywords=rehearsal+dinner+invitations
http://www.vistaprint.com/gallery/IEEAWQYBAAAAAAA=/invitations-announcements/rehearsal-dinner.aspx?&GP=3%2f26%2f2013+12%3a02%3a33+PM&GPS=2786786410&GNF=1
For our rehearsal dinner, we changed the wording around of 123prints wedding invitations:
http://www.123print.com/gallery/wedding/invitations/orientation/vertical
My favorites that I've seen are from wedding paper divas, but they're not always the cheapest option:
http://www.weddingpaperdivas.com/shop/rehearsal-dinner-invitations.htm
Other sites to check out:
http://www.shutterfly.com/cards-stationery/wedding-cards-stationery/rehearsal-dinner-invitations
http://www.zazzle.com/rehearsal+dinner+invitations
http://www.storkie.com/c-683-rehearsal-dinner-invitations.aspx
Photographer - Steve Belner, of Photovisions
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Our wedding has been featured on a wedding blog!