The Miniature Fox Terrier and Toy Fox Terrier are so closely related, it really wouldn't make much of a difference which one you got. Do you want a tiny dog or a small dog? The miniature does not appear to be recognized by the AKC however so I would question whether it is a real breed.
http://www.dogbreedinfo.com/miniaturefoxterrier.htm
http://www.dogbreedinfo.com/toyfoxterrier.htm
Here's the petfinder link to the American Toy Fox Terrier Club Rescue (they would be able to refer you to miniatures too)
http://www.petfinder.com/shelters/CA569.html
If you decide to go the puppy route, contact the secretary of the national breed club and ask for referrals to reputable breeders in your area. If the miniature fox terrier is truly a breed, the secretary of the toy fox terrier club can refer you to reputable breeders of miniature fox terriers.
American Toy Fox Terrier Club
Secretary : Michele Basye
Address: P.O. Box 702, Culpeper, VA , 22701-5941
Website: www.atftc.com
Email: mbasye@att.net
Reputable breeders and rescuers will be able to help you determine which size would be a good fit in your household. Ask them lots of questions and share your concerns with them -- they are very knowledgeable of the breeds.