wich are the best ones? how much? and where can i get them?

unless you have a turbo setup.

stock is just fine and anything else is pretty much a waste of money.

for new plugs go to Autozone or kragen's, tell em your car type, and get the ones that cost $2 or less for your engine. any company. the "Best" would be probably NGK's but Bosch and split fire are great too, just NGK's are japanese so it makes sense to put em in a japanese car.