Where do you find cases?
How do you store your collection?
Are there collections for sale?
Is there a site for collectors?

Please feel free to enlighten me.

Thanks.

Some of the answers depend on where you live (local laws)
Storage varies, but old (metal) blueprint/map cabinets work well, once your collection grows. You can start your storage as simply as a old cigar box, plastic storage boxes or wall displays.
You will occasionaly find collections for sale, usually when someone changes focus (starts collecting something else) or following the death of the collector.
Again, depending on where you are, there are websites for collectors, one being www.cartridgecollectors.org (home site for the International Ammunition Association). There is a forum there, with posters from around the world.
What your primarly interested in, should be where you start your focus (there are a lot of sub-specialities).
There are collector shows, the big one in the US is SLICS (Saint Louis International Cartridge Show), which is held the Thru, Fri & Saturday before Easter Sunday, in St Louis, MO, USA. There are also several shows held in Europe.
you can start by asking your shooting friends for samples of the various cartridges they shoot, along with asking other shooters at the gun range for a sample cartridge if they are shooting something you don't already have.

Behind the cannon rounds you can see a old blueprint cabinet

This one shows part of a wall mounted display (these look nice, but the cartridges like to fall off the shelves)

The following 2 photos will give an idea of what a cartridge show is like

from tables full of sorted and ID'ed cartridges to tubs full of misc rounds that you dig through and leave payment in a jar

Some good advice from Tailgunner .

Although he did omit to mention that cartridge collecting is highly addictive, mind altering , and potentially life-threatening ( " You paid how much for that ????? " )

But for all that its good clean fun , and the guys who collect cartridges tend to be unpretentious , down-to-earth folk as opposed to the snobbery that goes with gun collecting.

Be warned - the old shoebox you use for cartridge storage will soon be way too small...

have fun