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futurlec has them. If you ask, they will send you the specs.
Bought a ParkZone parking aid for $3 at a flea market a while back. There is a PIC 12C508(9?) that runs the show along with a precision voltage reference, comparator, op-amp, and such. Have yet to interrogate the Pic to get the pulse frequency and lengths though (send and receive). The single sensor one looks to be a Polaroid style. Those Polaroid cameras can be found cheap at thrift stores too.
Note to self: get it working, to turn on the sprinklers, and spook the neighbor's dog before he lays a large land mine again in the yard.