Usually you have to buy the gearhub(1), a rim with the same number of holes as the hub, some spokes, then assemble the wheel yourself… which requires dexterity, experience, a tool to cut the spokes to the right size and thread them, and a tool to "re-camber" (slang) the spokes.
Not easy, but
this service is super-brilliant: they sell a rear wheel already assembled with a Shimano GearHub 3-speed and it has O.L.D.=127mm, so it's already compatible with every bycicle 8/9/10 speed frame! you just need a couple of shims, +1mm and +2mm, to put on the ends of the axle!
Not too expensive, ~126 euro considering the exchange rate of the Swedish crowns in euros!
Unfortunately ... the company is located in Sweden, doesn't accept Paypal, and it seems they don't ship abroad.
But I love it! Want more services like this in Europe!!!

(1) e.g. NEXUS-INTER-3 , SG-3C41, 36H , O.L.D.=127mm , 3-Speed