Good ponies are hard to find! 
I don’t know what kind of budget you have but I would peruse the http://www.marylandponybreeders.org site. The members breed a variety of ponies–some breed classic Shetlands, some Welsh and/or Welsh crosses. If you contact the assocation I believe they have a listing of their members and the breeds they specialize in.
I notice they have a “Marketplace” on the site with ponies listed for sale.
There is also a Welsh pony breeder in Nottingham, PA–Guy and Helen Clardy–their farm is Clarwood Welsh Ponies and they have a website. I encourage you to call Mrs. Clardy just to talk to her–she may have a suitable pony or be able give you some leads.
I agree a pony that has been trail ridden would make a good candidate. Even if you had to put some extra training on the pony I think it would be worth it.
I found my Shetland via word of mouth, but the vet said he wasn’t a kid’s pony–not sure how he knew that, but I was in my forties and he was to be my pony so it didn’t matter.
There are Shetland type ponies on Craigslist from time to time but I think the best way to find them is word of mouth! Call the vets and farriers in your area and see if they know of any for sale. Also call any Pony Club or 4-H leaders in your area. Tack shop or feed store personnel may also have leads for you. Check The Equine Marketer, The Horse of Delaware Valley, The Paper Horse, and The Lancaster Farming Journal.