Hi Jay. Around here you can buy tridents at some of the more upscale garden centers. I like to buy them big at the end of the season when they go on sale and cut them down. You can build a pretty nice trunk very quickly that way.
Here's one I did:
http://www.bonsainut.com/threads/field-grown-trident.18131/
Here's a bunch that Brian Van Fleet documented on his blog:
https://nebaribonsai.wordpress.com/category/deciduous/trident-maple-deciduous/page/2/
And Al has documented the development of countless both here and on his blog. For example this:
http://www.bonsainut.com/threads/just-another-trident.18329/
Another option is to mail order something, but you'll be more limited on size (although it won't matter after a couple of seasons). I've ordered starter material from Brent - he's solid:
https://www.evergreengardenworks.com/acer.htm
And I've ordered seedlings from Matt - also solid:
https://kaedebonsai-en.com
If you're looking for something a bit further along, I'm sure that Don (
http://www.gregorybeachbonsai.com/index.html) or Bill (
http://www.internationalbonsai.com) can help you. In fact, if you can hold off for a bit you can go to the national exhibit in Rochester and take your pick. They have a lot of good vendors there.
Scott