The question "Where would I be able to purchase GMO seeds?" depends on who is wanting to buy them. A farmer, like myself, can go to a seed dealer and buy genetically modified corn and soybeans to raise on his farm. With that purchase there will likely be a contract to sign along with information on how to properly steward the technology. For the home consumer there aren't currently any GMO options I'm aware of that one can go out and buy from, say, a home and garden center. Bt sweet corn is available, but I think you would have to contact a seed dealer who works with farmers like me or some other commercial ag entity. You can't just go to your local lawn and garden shop or big box store and get any GMO seed at this time.
There is a GMO on the horizon that could possibly end up in your backyard. The American Chestnut tree. It has been devastated by a fungal blight over the last century. Researchers from the State University of New York are currently in the process of getting federal approval for these trees that combat the offending fungus by inserting a gene from wheat. It's a pretty neat project, and one that can bring back a tree that was once abundant in the United States. I don't know if and when they will be available for the average consumer, but it's something I will be following for sure.