Best place? IMHO, it varies. I hit all the usual suspects and see who:
1. has them in stock
2. has the best price.
Stay away from the off-brands and be sure to get Mec Gars. They are the OEM maker for CZ and others. Very good quality.
I can't swear on a stack of bibles, but I'm certain that they are a different part (SP-01 holds more IIRC.)