well it would depend on what kinda led's you use. I went the easy way and got them from a friend who bought 100 led's. The ebay seller sent him free resistors (for a 12volt system) with the led's so my friend just gave me 6 resistors 2 go with my led's. Teh best way would be to find an ebay seller that sells led's and gives free resistors (most of em do) and u will usually get it in less then a week from hong kong. You can also use an led calculator 2 find out what kinda resistor you need but then u have 2 go n find the resistor that will suit ur need. The one below is the best one and i use it all the time.
http://metku.net/index.html?sect=vie...calc/index_eng
You need to enter the specs of the led (all of em have it jst look for it) and enter it into the boxes. Use the single led one. Hope this helps.