Well, I just bought an old USB gamepad from a thrift store for $1.99 and soldered a 9-pin sub-D port onto it (Atari/Commodore style joystick port).
Works great and now I can use any Atari 2600 compatible joystick with any emulator (VICE, UAE, Stella, etc.)
So, I'm sure that something like those classic USB joysticks or a Stelladapter would work....they just do the same thing but cost more.
The only problem I had on occassion is that I had to set the "dead zone" in UAE's joystick settings to 10%, otherwise UAE always thought the joystick was pushing to the left. After I figured this out it worked awesomely!