Do you have a Brock White Contruction Materials outlet? I found mine there at a reasonable price... unlike one of the RV dealerships that I saw this morning, a 4" x 37' roll was 189.00!!!!!
Phone around to roofing suppliers, especially the ones that deal with EPDM or TPO roofing...chances are they might carry it. That's how I stumbled across Brock White.
have you tried online?....seems like a lot of good deals on it,as low as $53.00 for a 4" X 50' roll here's an example ETERNABOND ROOFSEAL WHITE 4" X 50' ROLL