Finding the transaction number may vary depending on how you purchased on AP. For example, if you purchased through Paypal, you should check your account there for the number. They may have also sent an e-mail with the number.
Regardless, the number isn't that important (it is simply to allow the Game Master team easier access to what the problem might be). If you do not have it, type "N/A" or something along those lines.
Also, in the case you are interested, here
is the list of common billing errors and their fixes.
Moved to the Billing section.