I tried to get an idea if they were planning to release the shirts fairly regularly,
but they pretty much blew me off.
(Which I took to mean that no regular release schedule is planned.)
Here's a partial list of what's been available.
Top, Left to Right
2.__Cartoon Cat and Bear__Kurtyboy (needs dyed to a light tan)
3.__TV Smoking Girl__Kurtyboy
4.__TV Blue Sun__Blue Moon
5.__Fighting Elves__Blue Sun Shirts
Bottom, Left to Right
8.__Pin Up Girl__Lilac Trading Station
My Firefly/Serenity collection here: