Where could I get some custom brass made? I have some ideas that I would like to play around with but can't find brass that meets the dimensions I want. Thanks in advance.

First, give us the specs and I bet someone here knows a piece of brass that will fit.
If not, Quality Cartridge has said they'd do most anything you can think of, but it will cost you....

Looking for brass that is 77, 88 or 99 mm in length. Just have a few ideas that I would like to play around with.

Well, there's a few that would fit your 77mm.
Do you have a particular case head size, or just length?
There's the .405 HDS Basic at 82.5mm or so, and a .455" base; and the 500 NE 3" is for all intents and purposes 77mm (76.2mm - ever *look* at .8mm?) with a .574" base.
For your 88- and 99-mm, you might have to look at the BMG.
Question is, what on earth are you going to do with a case that long? or better, WHY on earth are you going to do with a case that long?
Unless it is an extremely big bore, with a very long barrel, I can't think of a reason for it.
Is it a secret?

Give Quality Cartridge a call. They have made brass for me in the past.

http://www.owlnet.com/quality/

CJ