You should always be keeping records of this kind of stuff... and you should also be keeping up with iTrader... Like Mod said, your only option now is to buy a crack or get a lucky guess. There's a search function here which might be able to help you find who you bought the UC from.
I keep all my e-mails that I get from the PMs here - they're like a second record of things like this. I can just search them for the words of what I bought if something like this happens