Amazon or lots of other places online. They're usually called struts or props. Actually if you google something like "replacing Bigfoot
trailer rock guard struts" or something like that, you'll find the threads I was just looking at today. Not all about Bigfoot
trailers but it's all the same thing.
Mine are still attached just fine, but just like the struts that hold up the back tailgate window of truck toppers, they eventually lose their holding power and need to be replaced. I'll be replacing mine soon.
You may or may not be able to find an exact duplicate, but you can definitely find a replacement.
Easiest thing would be to remove one of yours and bring it in to an auto parts store or trailer/camper store. The length is the most important thing. Weight
is second most important, but it sounds like the originals were around 10lbs, and that's hard to find in a replacement. In some of the threads I was reading, people mentioned replacing theirs with 25 or 30lb struts and having that work just fine.
The slightly harder way is finding out what kind of attachment they have (ball socket?), and their length, and searching online for a replacement. Still pretty easy, but just bringing a part into a shop and having an "expert" find an adequate replacement is easier, I guess.