There is no standard "purchase agreement", they tend to vary region to region, sometimes office-to-office.
You can find many purchase agreements online, and at office supply stores (both online and in person)... but as everyone else has said... if you're going into a home purchase, please don't be penny-wise and pound foolish.
We recognize that you're likely trying to save money on Realtor's commission, okay... fair enough. But be smart enough to recognize that there are probably things that you don't know that could bite you in the hindquarters that would easily cost you more than the Realtor's commission. So at least, please, hire an attorney to take care of all the necessary paperwork, and guide the transaction to a smooth closing.