how can I get the receipt of my old purchases in particular application?thank you

If you no longer have the original email receipt for it then all you can do is try and find it in your account's purchase history and (depending upon what you need it for) either copy it's details or take a screenshot : log into your account on your computer's iTunes via the Store > View Account 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.