Shank found a couple on amazon that work. The Samsung Fit 128GB and Lexar Jumpdrive 128GB. other than that, you'll just have to test a heap of USB drives until you find one that works. I own 8 or so assorted drives and none of them work properly, had to order the Lexar one.
Got any info on the drive you have? I will admit postloader is pretty picky on the drives it decides to work with, but sometimes it's just a tricky formatting issue that windows doesn't admit to (I assume you formatted your drive with windows, not mac/linux). If you are running on a mac or linux machine I can offer some guidance that may help (I'm not sure how it works on windows, I haven't looked into how to do it)