where is the best place to get a ballast for a 400w hps

i looked at lowes but they didn't have the ballast but they had the light...or maybe i shoulda asked someone. what i am finding online is the ballast is usually around $100 plus shipping which ends up costing around 140...anyone know where to get one a little cheaper than that

also a question on Co2 - does spraying the plants during flowering with bubbly water do just about the same as getting a tank of Co2 - cause the Co2 tank is kinda expensive if i'm getting that ballast for my hps light

any suggestions are welcome

a hydro shop near you

HTG supply usually has some good ass deals on ebay, brand new, not used. Thats were i got my 400 watt mh/hps conversion system w reflector and free timer for only 204.99

good question, it seems hard to find any deals less than 120-140 with shipping. inside sun has some cheap hps packages though.