Ruger has discontinued their 10/22 magnum. Rumoured to be coming back at some time, but the redesign isn't being worked on right now. According to Clark Custom Guns site.
HK used to make one. Long dollars and not on the market for long.
Thompson Centre makes a .17 Mach 2, but not a magnum.
Volquartsen, Stateside, builds custom 10/22 in .22 mag. Expensive though. $1058.95US.
Other than that, like Grizzlypeg says, your options are a Remington 597 Magnum or a 597 Magnum LS HB.
Epp's, in Orillia, is listing the Magnum at $446.25. Shooter's Choice is listing a 597 LS Target at $575, the regular Magnum at $480. Both have them in stock too. Other shops will likely have them too, but you'd have to look.
Shooter's Choice is listing magazines at $14.95 each. No listing on Epp's site, but that doesn't necessarily mean they don't have any.