I have personally ordered San Marzanos from vendaravioli.com, salumeriaitaliana.com and pennmac.com. My favorites happen to be the LaRegina DOP San Marzanos, which I have only been able to find at Venda and Salumeria. The best prices are clearly at Venda. Venda also sells a lower priced DOP San Marzano called Famoso, which, at a little over $2 a can (the price I paid), is about a dollar a can less than the LaRegina DOP San Marzanos. They are more watery than the La Reginas (which has a thick puree), but the fruits are of nice color and taste. I would order them again. I have posted more than once about Venda, so you might find it useful to do a search on the forum using the search term "venda" (as an example, you should find this one:http://www.pizzamaking.com/forum/index.php/topic,1095.msg9756.html#msg9756). The best approach with Venda is to call them and to ask them which DOPs they have in stock and at what price.
PennMac is a good source for pizza ingredients in general, and among their DOP San Marzanos (I think maybe the only one) are the LaValles, in the 35-oz. can. I have had them, but I don't think they are as good as the LaReginas or even the Famosos. But that is just my personal opinion. If you choose to call PennMac, ask for Rose. Rose is a member of this forum and knows what our interests are. You should tell her that you are a member. PennMac's prices are better than most.
Both PennMac and Salumeria sell the LaBella tomatoes, but until we get clarity on what they really are, I would avoid them for now.
scott r is our resident expert on San Marzanos. He knows more about San Marzanos than anyone I know and has tried more brands than anyone I know. He is known for his many "San Marzano shootouts". Fortunately for him, but not for the rest of us, he lives in the Northeast and has access to some of the best brands of pizza ingredients around. My recollection is that he is not a big fan of the Cento San Marzanos (or the LaBellas for that matter) but is a huge fan of one of the other Cento tomato products (that may not even be San Marzanos). I suspect he will step forward in due course and give you some sound advice.
As a final thought for the moment, you might also want to check local Italian specialty food stores in your area. There are a lot more brands of San Marzanos around today than when I first started researching them, so you might get lucky and find some to sample.