Try gun shops, gun shows, egunparts
50-200 USD or so depending on specifics
20-60 USD or so
many of the European revolvers may have very limited parts available. You can check with gunpartscorp, and with Brownells (both have websites). However, the cylinder will likely require fitting and timing- it is NOT a "drop in" part- and may end up costing you as much as a new .22.
50-65 USD
50-75 or so
Inexpensive Italian copy of a Colt revolver. Value $75-$100 in excellent condition. Some also had a second cylinder in .22 Magnum- add $15 for second cylinder.