There is no such thing. But there are several things you can do to discourage snakes from entering your yard or home. -Keep your property clear of debris that can be used as shelter or basking areas. Large pieces of wood or metal are especially popular with snakes, as they warm quickly in the sun and provide quick cover from predators (like you). -Keep your lawn trimmed short and free of leaves or other cover. This will make snakes stand out, and discourage them from trying to cross the wide open swath of contrasting green that could be patrolled by their enemies. -Rodent-proof your property by keeping grains, fruits, dairy products and anything else rodents especially love well secured and not easily smelled. Check regularly for signs of mouse or rat activity, and set live traps around your home if you see any such signs. Do NOT use snap traps, poison or glue traps outdoors or in outlying buildings (including garages), as they are nondiscriminatory and can injure or kill non-target animals. -Keep pets contained with appropriate fencing. A pet in your yard will further discourage unwanted pests, and the fencing will help keep the pet from going off specifically looking for trouble from the local wildlife. -Find and close up any obvious entrances to your home that could draw in rodents or snakes seeking shelter. Any hole that's 1/4" in diameter is a potential entry point for pests.
repellents are gimmicks. Snakes are going to go where they get their needs met. Food, water, shelter, and a place to heat up.
There was a paper in Copeia in the 50s and some unpublished (I think) research at Tulane. Both concluded that the best repellant is the scent of humans.
None of that crap works
What about it?