Both old model, fluted, and new models, non-fluted with 22 magnum stamped on it, will work. They all vary sightly in overall length though.
So measure your 22 LR cyl with a caliper and buy one the same size or longer. If you get one longer just stone the front hub until it's the same length.
These are both old models and both fluted, mag on the right.
The new model 22 LR looks just like the one on the right but does not have shoulders in the chambers, the LR cyl does not. That's why the new model mag cyls are non-fluted, so you can tell them apart. Look for the shoulders in the chambers for it to be the mag cyl, old or new model.
There's a couple for sale in this forum's classified section yesterday or the day before.
Here's a new model on e-bay now:
http://cgi.ebay.com/eBayISAPI.dll?ViewItem&item=221161384092&category=73944