I would start with Camping World. I know they carry toilets and I beleive they have the tanks. Also look at RVUpgrades for similar stuff. In last month's Trailer Life there was an article about an RV salvage yard that harvests parts like this from wrecked RVs. Also, search the web for used RV parts. Hope this helps!
2006 Crossroads Cruiser CF32BL | 2012 Ford F350 Crew Cab Dually
Been to all 48 lower continental states once! Starting over...