Ask Mr. Smarty Plants

We are afraid we are not going to be able to help you very much. The palms are all tropical and sub-tropical in origin, and while they are often grown in "palm houses" in large botanical gardens, they are not suited for residential planting as far north as Pennsylvania. Adams County is in USDA Hardiness Zone 6a to 6b, and your average annual minimum temperatures range from -10 to 0 deg. F. That does not fall into "tropical or sub-tropical." Furthermore, to the best of our knowledge, none of the palms are native to North America but to the (what else?) tropics. We found an article  from which you might get some more information from Sun Palm Trees, Cold and Hardy Palm Trees. This article describes "hardy" as being able to survive down to 18 deg. F. Doesn't sound much like Pennsylvania, does it?

There is one tree, Sabal palmetto (Cabbage palmetto), native to North American coastal areas from Florida to South Carolina. That is not a true palm, and this USDA Plant Profile does not show the plant growing any farther north than North Carolina.

To move on to your other question about suppliers of exotic and unusual plant seeds; by definition, these are almost all going to be non-natives to North America. The Lady Bird Johnson Wildflower Center is dedicated to the growth, propagation and protection of plants native not only to North America but to the area in which the plants are being grown. When we Googled on "exotic plants," we got several websites, some of which do not sell seeds, and some of which will ship only to the West Coast, etc. One you could check out is WorldPlants.com Unusual Plants, Rare Plants, Exotic Plants. Unless you are planning to build a large, year-round greenhouse, we're pretty sure none of these are going to last more than a few minutes in the Pennsylvania Winter. Sorry.