The easiest way is for everyone who was compromised to list the suppliers they purchased from recently, then see if any coincide repeatedly. I have SERIOUS doubts that any of our approved suppliers would commit such frauds, but as most of the suppliers aren't really what I'd call big businesses it is possible that their server was compromised.
Now this I would be very hesitant to do. The "cure"/damage might be worse than the problem.
People's understanding of these things and how they work ranges vastly and is heavily weighted towards the clueless side. For example, why would you make the implication you did above? A great many people are going to think "supplier" as the source of the problem. If the list coincides with one or some suppliers it's only a vague indication of where to narrow the investigation. You ended by saying "their server" when it's unknown if any suppliers even do their own billing or contract it out to a service provider.
I'm guessing that's not what you meant to imply but that's the conclusion a LOT of people are going to jump to. As soon as you post a list of "suspect" suppliers you could do them serious damage when they might have nothing to do with it at all. Let's say 50 suppliers all use the same 3rd party billing service. Let's say your list (probably from a statistically insignificant sample of anecdotal reports) indicates one or two particular vendors. Let's say it was at the billing service where the data was compromised. Let's say the number of people reporting CC fraud purchased from some vendor during a period of time -- like maybe when that vendor was running some sale or contest or happened to have some popular product that was out of stock at the other vendors. And it was around that period of time that the billing service was compromised. Then you list that supplier as the source or just that people who purchased there were the ones that got fraudulent charges...
I would suggest, if you can get an ECF admin or Mod to do it, that lists be submitted to them to evaluate and investigate.
Another thing would be to get some information out here about these third party billing services. Who are they? How many of them are there? How does one evaluate or "score" them and then people might choose to only do business with suppliers who use the better/more secure billing services.
I *think* the way these things work is that the billing service provides the merchant with software/hooks for the merchant's online store/website. When you make an order it goes straight to the billing service and the supplier only sees the product order, shipping infor, and a reference number or possibly the last 4 of your card number... Something like that. Naturally the service costs money so suppliers might forego that and do the billing themselves while they are not equiped or setup with the proper security measures (also an investment).