Airam - hi, you may be able to clean the handles instead of buying new ones. Have you removed them from the faucet fixture yet? If not, the little cap on the center of the handle should easily come off with gentle prying (a small/thin flat-tip screwdriver or X-acto knife - be careful!), revealing the screw used to hold them onto the faucet. Once you remove the screw, the handle should come off. You can soak them in some hot water or even put them on the top rack of the dishwasher and clean them out.
If you want to buy new ones, you should make sure you have the faucet manufacturer's name (e.g., Moen, Delta, American Standard) when you go to the store. Taking the original handles with you is also a good idea. The big stores like H-ome D-epot and L-owes will carry many of the replacements. If your faucet is an older model, it may become a little harder to find the handles, so be sure to ask who may carry them. I had a devil of a time finding replacement handles for my mother-in-law's bathtub faucet fixture (an old American Standard). I had someone recommend a specific plumbing supply/hardware shop and those folks stocked them. I went in and bought a couple of them.
I hope this helps some - regards! Jim D/Heathsville, VA