Although I don't know what's available in the US, it's never been a big hassle for me to order Amiga stuff from Europe (I'm all the way down in Australia), if you don't mind waiting a few weeks.
In any case, the best control pad ever is the Competition Pro CD32 control pad. It has all the CD32 buttons, very nice sizes and layout, beautiful D-pad, auto and turbo fire switches for all the buttons, plus a slow-motion switch for the play/pause button. The feel of the pad is extremely comfortable, just like the original MegaDrive/Genesis 3-button pad (before it the tiny little 6-button one).
If these pads are too hard to find, there is a Polish group who are making new CD32 replacement pads, called the Boomerang CD32 Controller, which is the same shape as the 3-button MegaDrive/Genesis pad also, but using a layout more similar to the 3-button MD/G layout than the SNES-style CD32 layout, although it DOES have all seven CD32 buttons (four face, two shoulder, one play/pause). It also has a joystick and ball attachment for the D-pad, which is kinda cool I guess. Here's a link to the shop selling them, or they're available on eBay -
http://www.efunzine.com/efnz.php?a=pow&id=501&ra=pro&rid=501&sid=19dd9481b01f72ef924e6cfdff3dce9cNow, if you prefer joysticks, there are plenty to choose from and it's all a matter of personal preference and taste I guess. Personally, if I were to get a traditional single-hand joystick (non-arcade style) I'd want it to have 2-button support, as a LOT of Amiga games use a second button. Any joystick with Sega Master System compatibility will be fine. Usually button 1 is the trigger and button 2 is the thumb button, or the other way around.
And as stated before, any Sega Master System or MegaDrive/Genesis pad will work as a 2-button pad on an Amiga, although I would love to test this theory about turning one into a CD32 compatible pad. Has anyone done this, or does anyone have instructions on how to do it? I have a bunch of 6-button MegaDrive pads, and a couple of CD32s to test them on.