Even if you could retrieve it, it wouldn't help you. Any forensic evidence that you pull out of your own computer would be suspected of tampering. Any evidence you send them could have had entries deleted, or the events just might not have been logged, or you could have used another computer. You can't use key logs to prove you didn't do something.
Problem number two is that keyboard buffers don't timestamp entries, so even if you could get the things you typed, there's no way to prove when it was typed. A logger will timestamp when it got something, but trying to get it after the fact, there will be no timestamps.
BUT...
If they are handling financial transactions, they should be logging source IPs. Or, it you paid through some other company, such as PayPal, they should have logged where you were coming from when the transaction was made. If someone else was using your account, there should be some evidence that it came from another PC.
Unless they just used the PC that you did. Is there any chance someone has access to your PC and you just left something logged on?
Just some ideas. Hope it helps.