Yes, I found some HFT's used, tried them and kept them. I am using three on top of my dac and one each at the tweeter on my speakers. I use many so called snake oil products and I gladly spend my money when my ears hear an improvement in sound quality.
Some of the brands I use:
Synergistic Research: orange fuses HFT's, and one PHT(?) on the TT cartridge,
High Fidelity Cables, their MC-0.5 blew my mind when I first tried two and since then I am using DIY magnets as well to good affect, as well as an adapter in line with IC's.
Bybee Technologies: Quantum Signal Enhancers--similar to HFT's in that they are cumulative (the more you use the better the sound) and a real holy sh*t moment is their V2, expensive but powerful.
Tweek Geek's Quantum Science Audio: specifically their duplex outlet which I highly recommend, kind of like when you listen late at night when your system just sounds right, only more so!
IsoAcoustics: Gaia's under everything, speakers too!
At this point in my system, I think I am maxed out as far as tweeks go, and my system still has me going, "man that sounds so good!"
I don't know if I can tell you what tweek would work best for you, but most if not all of the above offer money back returns on your snake oil dollars! Tweek geek even let me keep the original (lesser priced) duplex outlet to A-B against the higher priced one--I kept the latter.
If you are basically happy with sound of your system and want to fine tune it then tweeks are in my experience mandatory (along with room treatment) to get the best out of you sound!
-Good luck!