I buy mine for herehttp://www.cut2sizemetals.com/?utm_source=MadMimi&utm_medium=email&utm_content=Our+July+4th+Schedule+-+please+read%21&utm_campaign=2012+July+4th+Schedule&utm_term=Cut2SizeMetalsA tip if you order from here. On their order page click the box "will accept larger if available" if they have a bigger piece than you wanted already cut you will get it for the same price as the smaller.
For the few thicknesses they have the price is definitely low... question is what the alloy is? They don't say online. The price is worth a call.
The last time I needed sheet brass I just went to Lowe's and got a brass kick plate for a door. You can find them in the hardware dept. I think around 6 or 8 bucks .