a 3/4" UNF (Unified National Fine) thread is 16 threads per inch. You're about as likely to find a lefthand die in that size in a local parts store as finding a chicken with lips. The place I go for machine tooling and equipment and parts is Victor Machinery Company. (www.victornet.com). They have a minumum dollar amount for an online order- - - -don't remember what it is, but it's fairly low. I'd suggest going online somewhere like Ebay and buying a thread pitch gauge. They're available in both inch and metric sizes, and every tool box should have a few items like that. Ditto for a dial or digital caliper, preferrably one that can measure in both metric and inch dimensions. Like it or not, metric is the common measuring system, and America was at least 50 years behind the rest of the world in switching to metric measurements. Everybody else did it after WW II.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!