The LH in Weimaraner is a recessive trait naturally occurring in the breed. Unlike the blue Weimaraner, the LH variety can come from well-bred show or field stock. Because it is a simple recessive trait, every SH Weimaraner has the potential of carrying the LH gene.
The AKC standard describes long hair as a fault so no in in the United States is deliberately and actively breeding LH Weimaraners. Finding one that is well bred may require a lot of sleuthing and a willingness to wait for the right puppy. Breeders of shorthaired Weimaraners often get LH Weims in their litters, finding these puppies won't be easy.
Start with your local Weimaraner club and branch out from there. There is a list of all local Weimaraner clubs at Weimclubamerica.org - contact as many people as you can. You will likely have to travel to where the LH is at.
BEWARE of anyone touting their rarity, asking exorbitant prices or breeding them in conjunction with other "types" of Weimaraners like blues, longhair blues or brown Weimaraners. Those with the best interest of this breed in mind won't be involved in "recreating" the breed.
Don't compromise the basic rules of buying a healthy dog - just because it's a longhair. The parents should be OFA certified, come from stock that's actively showing or participating in field trials and the breeder should be a WCA member.
Follow these links to learn more:
http://www.weimclubamerica.org/worldweims/longhair/index.html
http://www.weimaraners.com/coats.htm