+1 to having a wheel built and a mating crankset isn't particularly costly either.

Depending on function wanted there are a bunch of choices

NuVinci N360 - planetary gears, neat
Rohloff - The Rolls Royce, about $1300
Alfine - multiple choices for number of speeds/range
iDrive (I think, the SRAM one)
Sturmey Archer

There are shifter options beyond the stock grip shifts for a few of those too, based on bar configuration etc. A few for the Rohloff and Jtek for a bar-end for the Alfine as examples.

Likely others...and tons of info on those above searching this site for sure.