"If you are a fan of Lemons and Demons (or, you freak, of Lemon Demon) or are a shirtless person who stumbled upon this website in a desperate search for something, anything, to clothe your nakedness so that they'll let you into the convenience store, this is the right place for you!" -lemonkle.com
Once upon a time, until quite recently, Lemon Demon t-shirts were available for sale at lemonkle.com. Now that website is defunct and the link has been removed from the main LD website. It is unknown whether or not new t-shirts will become available in the future. Neil did say "more merch coming" upon the opening of his Topatoco shop, which currently sells only posters, but that was nearly a year and a half ago, so who knows.
Anyway, the t-shirt model photos were really good, so I have saved them here for posterity.