If you log into your account on your computer's iTunes via the Store > View My Account (Store > View My Apple ID on iTunes 11) menu option, you should then see a Purchase History section with a 'see all' link to the right of it. Click on that and you should then see a list of your purchases, which are sorted by date and show the iTunes order number (I'm not sure if in-app purchases and/or auto-renewing subscriptions show there).
If you can't find the item, and/or you don't think that it's yours, then you can contact iTunes support via this page : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption