REOs don't use tanks. The REO is topped with a BF RDA. The REO has an on-board bottle that the user "squonks" liquid up-through the connector into the atomizer.
Not trying to be Nazi, just clarifying your terminology.
There are several BF-ready RDAs for sale on the market. reosmods.com has several to choose from. Almost any RDA can be modified to use with a bottomfed mod. There are some exceptions, but few. All the atties on reosmods.com are modified/built for use with the REO.
Good luck and we'll see you in reoville soon!