I need help finding a water barrel big enough. 

There used to be a local seller who sold "barrels" that were 4'x4'x4' cubes.  I finally put in a garden so I had something to do with the water, but they went into business making actual barrels and no longer sell the cubes.   He pointed me to F&F but they have livestock watering tanks, which isn't at all what I wanted.

A dinky little barrel will fill in something less than 30 seconds in a moderate rain.  We only have 4 rain drains and the one I want to put the barrel on gets more than its share of roof draining into it.    I looked around but couldn't find a bigger one for sale elsewhere either.

Anyone know where I can get a BIG rain "barrel"?

I don't know about a specific seller, but I've seen similar water tanks listed on craigslist in the farm+garden section.  This is in my local area, but they may have some near you as well.

How pretty do you need it?

Plastic garbage cans work well and the plastic is usually UV-protected, and usually have a tight-fighting lid.

A nearby auto repair place occasionally gives away on craigslist 55 gallon steel drums that contained antifreeze, I've used them for a number of projects.  One, unintentionally but due to a small hole in the lid, as a rainbarrel.  :-)

Also, it's possible to chain the barrels together, if you can't get a single container in the size you want.  Just hook up the overflow pipe from the first barrel as an inlet to the second one.  You can connect as many barrels as you want.  Just be sure that the last barrel in line still has an open overflow pipe.

Here is an example:  http://www.instructables.com/id/Rain-Barrels-1/

what your looking for is called a tote. usually 40X48X40 and hold 300 gallons, theyre available in 275, and even 150 gallon sizes as well.. theres a scrap yard here that occasionally gets them because the cages are metal. many manufacturers use them in some capacity or another, and often they can be had free, or for the deposit on them (usually $75).  I worked at menasha company in Wis for a time, and we got our glue in them.

many larger farms will get teet cleaner or soil treatments in them as well, looc around for a company that manufactures some product, papermills, cardboard places, assemply plants, packagers etc.

if this is for watering edibles, or potable use, be cautious of anything labeled toxic. most can be cleaned if you know how though, most that contained adhesives can simply be steam cleaned.

easiest solution; ebay.. be prepared to spend over a hundred though.

=:3

a very good point, i dont like soap barrels though, almost impossible to sterilize and fully clean. i use a few for storing oil though. again look to farmers for 55 gallon and 30 gallon barrels. i have one locally that i get mine from, usually $5 a piece.. though last time it was $10..