First of all, it takes VERY little power to tumble a tank. A 1/15 HP motor (like a sewing machine motor) is more than enough.
Second, you want low speed, so you really want a gearmotor, not simply a motor. If you can direct connect the output of the gearbox to the drive roller (maybe with one of those spider connectors that have a rubber piece between the input and output hubs) then you want the rated output speed of the gearmotor to match the fastest speed you want on the roller. I'm guessing this is a few tens of RPMs. If you plan to link the motor to the roller with a belt or chain, then you can go up to 5 times faster or slower.
Lastly, the easiest motor to speed control from 120VAC is called a Universal motor. These motors are used in sewing machines, hand drills (corded, not cordless), shop-vacs, etc. You can just hook them to a standard lamp dimmer from the home store.
Suitable motors cost hundreds of dollars new, so I imagine you want a used one.
An example of what I'm talking about is eBay item 310164163164.
With the addition of a bridge rectifier after the dimmer, you could use a 90V DC motor. These tend to be nicely made, and more expensive. An example is 320417910472.
If you're a cheap bast...d like me, maybe you want to try this:
3 OUTPUT GEARMOTOR ASSEMBLY-MPJA, Inc.
There's better availability of cheap surplus 12 and 24 volt DC motors, but these require a power supply to run off the AC line. The 9902 PS at mpja.com would be suitable to run a small 12V gearmotor like their 16779MD.
Don't hesitate to ask if you want advice on anything specific you might find.
D
