Instructions
Decide how much you like the game. When it comes to dance pads, the adage "You get what you pay for" is true. The more money you are willing to spend, the better the dance pad you'll get.
Buy a soft pad if you are only casually interested in dancing. Soft pads are the cheapest, but they also wear out quickly under heavy dancing modes. But they'll get you through the trial period while you decide how much you like the dance game.
You should be able to find a serviceable soft game pad for under $30 at any game store or most chain stores that sell electronics (Best Buy, Target, Circuit City and the like).
Buy a hard-foam pad if you are dancing for exercise. The foam is thicker and works well for everything but heavy dancing. Hard-foam pads are more expensive than soft pads (around $120) but they last longer, are more responsive and don't slide around during play (as soft pads tend to do).
Find a hard-foam game pad online or at a dedicated game store. Redoctane (redoctane.com) sells an excellent pad called the Ignition that is widely considered to be the best in hard-foam pads.
Buy a metal pad if you plan to dance in tournaments or prefer a heavy model. The metal pads are expensive ($150-$300) but are far and away the best in terms of responsiveness, durability and play experience.
Find metal pads online or through your local game store. You may have to ask your game store to special-order your metal pads; most game stores only keep soft and hard-foam pads in stock.