you need to go to a grow shop to find an hps bulb and ballast, dont forget you need one of those too!

these two websites sell hps

www.bghydro.com
www.1000bulbs.com

you could also look on ebay for a used system for cheap, if money is an issue

as for ordering asap... if youve got the cash, id get a 400w MH system right away for vegging... the hps can wait until you flower