I bought a stainless steel pad off eBay-one thing I wish I had checked for however was if the panels are offset from the rest of the platform-either raised or sunk somehow. Now when I play with mine I can't tell if I'm off the squares until it's too late. As I wear it out though I've noticed the sensitivity has gone up and it's easier to have it respond to slightly off presses.
Also the pad should have a USB connector if it's PC compatible, and since we're talking about pads they should be able to naturally handle two presses. If you buy PS2/Xbox pads and a converter, make sure the converter can handle 2 keypresses, or has a switchable "Dance Mode" that allows simultaneous keypresses.
Highest quality pads are Cobalt Flux or RedOctane but with high quality comes an expensive tag. The only games with corners that I can think of is the one Paige used to play-Pump It Up I believe? but that also required a center square, and only Cobalt Flux pads have that as well if I remember. Many pads have a control box that allows switching between PS2/XBOX/PC so I'd look for that.
The bottom two that are missing are the equivalent of O and Triangle in PS2, at least on mine, so you can get away without them by using select, but they are nice to have.