Something to be aware of, Dorothy; some 620 cameras won't take up on a resized 120 spool like the ones J&C sells. Some will -- my 620 Argoflex works just fine with 120 on both endds (after converting the supply side) -- but on others, like my 620 Kodak Reflex II, the drive key on the advance is too small to catch in the 120 spool (especially the tapered end Kodak type spools -- if you have trouble, try to get some resized film on a Chinese or eastern European spool, or post back and I'm sure someone here will find you some 620 spools for takeup).
Also, some/many 620 cameras (including my Reflex II) will very happily accept 120 supply spools with only the flange diameter trimmed (about $5 cheaper than resized, if you carry a pair of heavy nail clippers or similar), so if you have a supply of 620 takeup spools, you're independent of both respooling and resized film.
However, respooling is a good skill to practice, since there are always a few cameras, like my Savoy 620 and Ansco Pioneer 620, that won't accept any kind of trimmed or resized 620 spool, no way, no how. Five minutes in a changing bag or darkroom, and I can feed them any time I like, as long as I have two 620 spools handy.