According to Richters, it does not come true from seed.

"

"Curly Mint" Seeds

Hmm, curly mint...
now there’s a good case of caveat emptor. Crisped leaves in Mentha are
under the control of at least one gene, and another gene probably
controls the degree of crispness. Crisped mints are known in spearmint
and peppermint, its hybrid. Both these have been called curly mint, but
the former does not breed true from seed and the latter is intensely
sterile. The best scam of "curly mint" seeds that we encountered was one
packet labelled curly mint on the front and peppermint on the rear
accompanied by a picture of the green pineapple mint, M. Suaveolens. It germinated to a rank, plain-leaved spearmint! Other packets of "Curly mint" germinated to M. Suaveolens. If you want to gamble, here’s a way that’s cheaper than Las Vegas or Atlantic City."

Thanks Susan, and I expected such was the case. Now if only I could get a time machine and go back to either of my grandparents garden...I'd cut a few sprigs of doublemint and hurry back to present day and let them root so I'd have some of the great mint. I have always loved mentha and my mom loved curly mint and I'd try to find (to no avail) plants for her. Had no success, but mom kept saying she knew it was out there, and loved it. In my mind I can recall both of my parental grandparents gardens, but can't remember where they planted the herbs they used in canning or fresh to the 'garden table'. Yes, the huge solid oak table was always referred to as the garden table. They had huge Missouri claysoil gardens, but managed to live off of the gardens while raising large families and helping those that didn't garden.

That is a wonderful story; I hope you are able to get your hands on the plant. It looks like Debaggio's Herb Farm, in Virginia, carries what you are looking for, but they don't do mail order. If you know someone down there, maybe they could buy some and ship it to you?

"Mint, Curly Or Crisped. Mentha xpiperata.
This mint first came to my father with a whispered story of intrigue, a
"double mint" plant smuggled from France. True or not, there was no
reason to smuggle it through customs; it has been in American herb
gardens for decades with good reason. While technically a peppermint,
it has retained the odor from its spearmint parent. It has decorative
puckered leaves with crinkled edges that recommend it as a garnish. It
is also nice in mint juleps and iced tea."

Good Luck!