I'm suing my Ex-roommate, but don't have receipts

This case seems like it will basically come down to your word versus your ex-roommate's word.  You must testify to what you wrote in your email, bolster it with whatever other evidence you can, and hope that the Court believes you.  You call your ex-roommate to testify about your agreement, and hope he/she doesn't lie, or if he/she does, that the Court does not believe him/her.  You call any other witness to either the agreement or the theft, e.g. a neighbor who saw the clothes thrown on the lawn, a friend who was there when you reached the agreement, etc. (it may be difficult to get them to show up to Court, even with a subpoena, but do what you can.)  If you paid by check or credit card, maybe you can introduce statements to show your expenditures and tie them in to particular clothing or other stolen property.  Maybe you have some photographs which show the stuff thrown on the law, or the contents of your closet, or show you wearing some particularly expensive stuff which was stolen (a diamond necklace or a fur coat, for example., or maybe there are people who say you wearing the diamond necklace shortly before the theft who can testify that you owned it.  In short, any proof that backs up your story will be helpful.