It looks like the diecast tanks for the older TMCC Geeps are "no longer available," at least at some of Lionel website parts areas I looked at for those engines. However, I think these tanks can be found on most of the newer LionChief engines, including Geeps, Rectifiers and RS-3s. They all use the same tanks; they may have different openings on the bottom for different slide switch placements, but otherwise they're the same and could be used, I think. That part no. is 630-8824-214, and is available for any of those engines at the Lionel parts site. Another possible choice, which seems like it would be the same as that on the older TMCC Geeps, is part no. 610-8579-506 (see parts for the Rock Island GP-7 No. 28536, for example). The notation by that part says it's available from North Lima Trainworks.
With this lowered tank, and with the low stance of these Geeps (they're scale-sized, incidentally), these engines really look great, and they look good with scale or traditional rolling stock. I've not seen any other engine, scale or otherwise, that sits any lower and with less space between the trucks and the chassis.