If you have some type of print shop you can just make your own. At the top of them just put Thank You Volunteer and then you could have the people they volunteered for sign it, then it is very personal. If you do not have a print shop here are a few sites that may be helpful, http://www.freeprintablecertificates.net/category/appreciation, there are really nice ones here that are free to print and
. here is one more http://bulletin.spps.org/20APRIL200707.html, this is a school site that uses some but you can download them as well. Hope this gives you enough to choose from! Dont forget to personalize them before you print them!