Not cheap, but if you want the best experience get the 100k Happ Analog Trigger Stick (15 pin connector).
Add a $10 usb gameport adapter and you're done.
You could interface a standard used 5k one using a U-HID or A-PAC, but in my experience(hacking pads or wiring to a gameport)
the pots with a wider range of resistance result in more precise, smoother movement.
I have not tried them with a U-HID or A-PAC, so they might be fine with that.
Looks like they have a wide variety now, including a pretty sweet looking flight stick that's double the price.
http://na.suzohapp.com/wp/search.p?SECTION=ALL&SEARCH_TXT=analog+joystick&Search=GOI have a used pair that I swapped standard linear 10k pots from radio shack into. It's connected to a hacked gamepad.
It works, but not quite as well as I want it to. The Happ pots are specially made to cover their entire 0-100k range through the small range of movement the joystick allows.
I'm only using a small portion of the 10k range of the radio shack pots. I tried to track down an alternative source for the 100k Happ pots, but the part # stamped on them leads back to "custom built to order" on the original manufacturer's website.
I bought one Happ 100k pot just to get the part # off of it. Eventually I'll probably give in and order a 2nd one so I can have one 100k triggerstick.