There is no such thing......they are all gonne require a major hassle.

Any tank will require the relocation of the mounts on the tank or frame, relocation of the speedometer, relocation of the lights in the tank dashboard, possibly width modifications to the tunnel, and even relocation or adaptation of the fuel petcock.

One potential alternative is that you were previously considering saddlebags/paniers made from ammo boxes on your bike.  You could make a tank out of one of these (or a portion of one of these), and install a small electric fuel pump to run fuel up to the main tank when you needed.

Another potential cure is to take a stock tank and widen the sides and/or make it taller in the back....while leaving the bottom and mounts and speedometer intact. This has the potential to be very ugly or maybe even acceptable.....just depends how artfully it is done.  At the AMA museum there is a little Harley 2 stroke that was one of the first small bikes to challenge the large 4 stroke bikes in enduro racing.  The owner took the small peanut shaped tank and extended the back of the tank up to increase capacity.  Being a race bike they didn't even flatten out the welds......and it looks.....functional.

I don't know if this picture will show it.....but there is a triangular wedge piece welded in at the back of this tank to raise the back of the tank up and make it hold more fuel.

Honda CB tanks can be close. So can Kawasaki KZ and Suzuki GS tanks. Some will use the stock mounting pillow bar locations (the big ugly rubber things), some will require you cut them and drill the frame for a Harley Softail mounting bolt and acorn nuts.

My tank looks like a modified Honda ATC (outlawed 3 wheeler) tank. Mounts in the front like a Softail tank.rear mount is a tab tht is captured by the stock front seat mounting hardware. BUT, you can see all the crap Suzuki was too lazy to hide in the backbone/gusset pocket, ie, the coil and decompression system. And I had to toss the right hand chrome piece from the cylinder head (good riddance) for the Rebel 250 style petcock to fit.

Straight up swap? Forget it. My next tank will require a frame stretch, I'm sick of micro tanks. If it is under 3.5 gallons it isn't worth the hassle.

There are several threads going here in Rubber side down about tank swaps right now. Do some poking around.

One guy here used a 1970's GT550 tank and said it was pretty easy.
Another guy used a 1980's GS450 tank I believe and said the same.

Mine has a '69 T350 tank, and my now ex-"friend" who was working on it chopped up the frame to make it fit, but now that I have the bike back in hand, it's obvious that it wasn't necessary, and in fact has caused me more problems in other ways. I could have done it with some simple modifications to the tank and not touched the frame at all, but it would still have required some hammering and welding on the tank, plus I have to remove the Decomp solenoid and re-locate the ignition coil, but that's not really a big deal to me...

I've seen Harley tanks used too... Pretty sure that guys said it was a bolt-on, or simple to mod kind of thing. His bike is yellow, that's all I remember...