Corwin-Arms:
http://corwin-arms.com/product/svt-40-tool-set
The stripper clips are the same as Mosin stripper clips:
http://corwin-arms.com/product/mosin-nagant-striper-clips-5-pack
The only difference between a Mosin cleaning kit and the SVT-40 cleaning kit is the gas adjustment tool and the sight tool that also doubles to remove the barrel locking pin from the stock.
Both are included if you get a proper mag pouch for the SVT:
http://corwin-arms.com/product/svt-40-tool-set-and-svt-40-two-magazine-leather-ammo-pouch
Spare magazines are hard to come by. Just got a couple of repro mags from Trade Ex, and they're going to need a bit of filing to get them to fit and feed right, but I don't mind the application of a bit of elbow grease:
http://www.tradeexcanada.com/content/spare-magazine-tokarev-svt40--
If you want spare mags, I would suggest to jump on them. They kind of come and go, and you can go easily half a year not seeing any of the dealers having them. Although that might change, and the quality/price improve, now that so many SVT's are flooding the market.
The big ticket $$ item will be tracking down a Bayonet for it, if you want to build up an "as issued" SVT rig. Expect to pay $250 or more for a bayo and scabbard.
A proper SVT sling is the same as a PPSH sling, and they're fairly easy to find. Mosin slings are also "period correct" as they were often substituted during the war, depending on production pressure.
The ideal "as issue" kit would be:
Mosin cleaning kit (same tools)
Gas adjustment tool.
Sight tool/barrel retention bolt removal tool
PPSH style sling
Bayonet w/ scabbard
Mag Pouch (different from the stripper clip pouches that the dealers tend to send out with the SVT's)
2 spare Mags