I need advice on lighting supply store/ correct wattage

First i would love if anyone out there knows a good site for getting cheap HPS,
im using about 4 square feet to work in. im wondering if 400 hps is too much?

19x31,

www.bulkhydro.com best prices on Lumatek ballasts. Get a dimmable 250/400. Your space kinda falls inbetween the 2 wattages, so....this would give you some flexibility.

would all dimiabble digital balast be able to plug in a 250 watt bulb?