If you're okay with small ones, Lowes and Home Depot carry them.
Lowes sometimes has a $30 bucket of them that often come with 1"+ trunks but miserable nebari. (know from experience!)
You'll need to put it in a bonsai pot on your own if you really want it to a be a bonsai.
I really wouldn't rule out getting one delivered from bonsaiboy, Brussel's, or Wiegerts. My experience with jades is that the branches aren't really all *that* fragile and anything that breaks off can be easily regrown on the main tree and the part that fell off regrown as a cutting. More trees!
Ficus, Schefflera, and true Crassula Jades are also viable options for someone with just a window. Those can also be bought at Lowes for less than $20 in appropriate sizes.