Google:
"Salvage Mustang Super Snake"
Or you just have to monitor recycler and salvage facilities like CoPart, IAAI, AutoBidMaster, Parts Farm, Mustang Parts Specialties, Prestige Mustang, Salvage Bid, Erepairables, Auction Auto Mall, etc....
But don't get your hopes up, because niche Mustangs like those - those facilities are very well in the know, so an S550 Super Snake Wide Body won't be going for pennys.... In fact, even if parted the parts alone will be worth big $$$$ as well. Not to mention the cost will be high if the SSWB Chassis plates are still attached too, because those usually go missing due to trophy hunters/parts poachers and they usually cannot be replaced.
Check eBay too, very now and then niche Mustang parts surface in there.
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One more note:
By the time you would have picked up a totaled out SSWB, then having to spend $$$ fixing damages, plus the $$$ to repurpose as a drag car, you could have spent $$$ on a used S197 or S550 Cobra Jet that will not be wrecked, have everything for immediate drag racing and turn really quick times right out of the box. You can find them in the $75k-$100k range depending on over all condition and if they have race history (or not). You'll have more into trying to fix a wrecked SSWB than that... because if the specific WB panels are damaged or missing, you're going to have to get them form Shelby American for the genuine parts - unless you go with some aftermarket WB kit that isn't the same as the genuine parts. I think the S550 WB package is in excess of $40k alone (back in S197 days it was a $28k "option" on top of just a Super Snake conversion $$$).
Good luck with your searches and post up a build thread if you do find one and the intent is to do what you speak.