Currently the cheapest ram (as far as per gig is concerned) is 2gig sticks. You could add 4 of the 2gb sticks and get your 10gb. (2x1gb + 4x2gb = 10). This would also leave you with 2 empty slots. Search for some of Multimedia's posts on how to best arrange your RAM.
OtherWorldComputing has always gotten good reports. They aren't the cheapest, but it's quality stuff. They also offer a trade in program you probably won't take advantage of.
If the RAM is from a reputable dealer who certifies it for the Mac Pro it's all going to be about the same.