There are at least 3 different types of .22LR conversion kits, and all of them take different kinds of mags.  But the most common is the Ceiner kit, and it is the best IMO.

The problem is that the 30-round Ceiner mags are very hard to find, as the only source for them is out of someone else's collection, and current prices for them hover around $125-150 each, when you can find them.

The day that Ceiner ran out of these, the price nearly doubled from the $75 or so that they cost through the end of the 90's.

-Troy