Where to buy:
Note that the prices vary quite a bit depending on what color barrel you choose. The barrel comes with a hose bib (spigot) on the front so you can use a hose with it. You'll note from the picture that I have placed the barrel on some blocks to elevate it so that I can also fill a watering can from the spigot.
The offset diverter allows you to control whether the water goes into the barrel or into the downspout. It's not a necessity, but it's best if you don't allow the first water that comes off your roof when it starts raining to go into the barrel.
I bought the other downspout parts I needed at Home Depot
Rebate applications can be filed via email or snail mail. Be sure to read and follow all instructions, i.e you must take before and after pictures from 2 angles.
Note that these links are for rebates in the San Diego area. You'll need to do a search for rebates in your area.
$75 per barrel
$1 per gallon for rain barrel