You can do a Yahoo or Google search for "9mm brass for reloading" and come back with a plethora of sellers.
For "new", unfired cases, I have bought from the following:
You can buy new casing if you want, but most reloaders use previously fired cases (they are referred to as "once fired" but unless it is military brass with a crimped primer or you know the shooter, there is no guarantee as to how many times it has been fired).
And in the "once fired" market, you can get cases that were just picked up off the range and sorted by caliber all the way up to cases that have been fully processed and are ready to reload. In general, the more processing the seller does, the more it costs.
You can also buy cases that have been "de-militarized" (also knows as "pulled" or "broken down") which are taken from a batch of ammunition the manufacturer had to disassemble because it failed to meet specification. Frequently such cases are available having already been primed.
I have bought various grades of previously fired and de-mil cases from the following sellers without major problem:
There are other sellers, but I haven't dealt with them or their name wasn't available off the top of my head. Remember when buying brass to factor in the cost of shipping. Some sellers price their product with free shipping, for others shipping is additional.
Among previous fired brass sellers, the count of cases is usually determined by weight, not a physical count. Most of the time the weight is a little heavy so that you get a few extra cases to compensate for the occasional damaged case or berdan primed case so you may order 100 cases, get 104 and when inspecting them find two damaged and three berdan cases so that you end up with 99 usable cases. Some people would consider this a problem, I don't. If I ordered 100 cases and only got 50, that would be a problem, but not 99.
Also, if you are going to start using previously fired brass, I suggest you get a dedicated decapping die since you will eventually encounter a berdan case that both the seller and you missed and you don't want to have to end a day or reloading prematurely because you broke your last decapping pin.