... a couple different types.

One is the olive barrel type cited in a post below -- my particular vendor is mirainbarrel.com (MI -- he is in Michigan). These olive barrels are from Greek ships and are NOT certified for food grade storage in the US -- therefore they can't be used for much here. Except... rain barrels! These are 58 gal capacity. They are "terra cotta" in color and look very nice design-wise. The walls are rather thick and they seem very resistant to freezing/thawing stresses, so I just open the valves in the late fall (western MD) so not too much ice accumulates. Pros: color and design looks great, screw-on sealed lid. Cons: overflow too small.

Our older barrel system has white barrels, and they look like the typical cylindrical 3-section 55 gal sealed oil drum. Plus on this model is it has a very good overflow system. Con is... it's white and therefore somewhat translucent. Algae can develop -- though that's certainly no problem for a garden.

For both of these barrel types I prop them up the same way:

1) set down a 16" concrete paver about 8" from the house wall
2) stack TWO cinder blocks on each side of the paver.
3) Cut pressure-treated 4x4s into 32" lengths and span your cinder block stacks with those to make a strong, nice looking deck. You will need four per barrel. Leave 1/2" between the 4x4s for drainage.
4) Set your barrel on top. Two blocks plus a 4x4 of height is plenty of clearance for watering cans.
5) Use plastic flex adapter pipe to connect your downspout to the barrel top. If your barrel doesn't have a wide overflow (like the Greek ones) use a diverter also.
6) Now set another 16" paver in front of the first one. This will make a good platform for filling watering cans.
7) Add quick-release hose fittings to the barrel hose bibb to change easily from hoses to cans without screwing and unscrewing.

Pressure? 24" of height is about what I get from this, and that's enough to trickle through a 100' hose across my flat yard. If you want real pressure, you need to put the barrels on a (very strong) rack about 8 feet high. Or... if your site is hilly and your house is maybe 20 feet above your garden location, you'll get about the same pressure you have in your house plumbing.

Storage volume? Both of these barrel types are easy to connect in series. I've seen pics of folks lining up 10 or 15 of these by the end of their house -- that's 500 gallons or so of storage. But I only use four. To make a real doomer water system like some people try would get pretty pricey. My four barrels will water my garden all summer, unless we really have some drought.