We would usually use an article (a, the) or a pronoun before "signed." Without such a word, it sounds a bit strange. Here I think "a" or "our" is what you want. And probably "our" is best, if you are sending a copy of your own confirmation of purpose. Enclosed please find a copy of our signed confirmation of purchase. You could also say "purchase confirmation" instead of "confirmation of purpose." I think this sounds more natural, but many times business language is not natural sounding. It has its own, stilted vocabulary.

It would be better to write: "A signed copy of the confirmation of purchase form is enclosed", "Enclosed is a signed copy of the confirmation of purchase", or "Please find enclosed a signed copy of the confirmation of purchase." This link provides related information: http://english.stackexchange.com/questions/43922/how-should-please-find-enclosed-be-used