How do I find a smaller tank for my VTX1300?

I don't know how well aftermarket serve the VTX. Try and find online owners forums... If not you may be able to find an aftermarket tank destined for a Harley that will fit, and get the mounts modified. Alternatively there are guys that will fabricate custom tanks, but you can guess they ain't cheap.

The BMW GS had two possible tanks. The model with the gigantic tank was called the Paris Dakar, or GS PD. The normal tank model was a plain GS. Never heard of any other bike with two tank sizes. Custom tank, I'd say.

1) join an owner's club

2) ask on their forum

3) buy the recommended replacement

4) fit it.

https://www.vtxoa.com/

There will be others

Take some measurements, go to a motorcycle salvage yard and start picking.

But remember- if your rides are further down the road than the nearest pool-hall than fuel stops are more often.