Back in the mid-late 90s there was a guy on the Turbo Mailing List who picked up a bunch of NES Advantage controllers when Kay-Bee Toys was unloading them on clearance for something like $2-3 ea and he converted them to Turbo or Duo controllers for $15 shipped. I picked one up from him bitd and it was my controller of choice for a long time.
Unfortunately, I had a bout of stupid and sold off my entire collection a few years later. I regretted it almost instantly and started rebuiling my collection after only a handful of weeks, but was never able to hunt down another Turbo-converted Advantage. I would guess that it's not too hard a mod to do myself, but I don't have an extra TurboPad to rip the cable from (though I do have an Advantage that's ready and willing to make the trasnformation!).
Also when I sold my collection I had one of those Hori 6-buttons joysticks (The black PC one) and it was AWESOME.