I seriously doubt that there's a "shortage". There's nothing about a brown ale recipe kit that would make it different from other kits. Most likely, there's just far less demand for that style. If you're buying milled kits, it'll definitely be a question of demand and not supply. Homebrew suppliers aren't going to stock milled grain products if they're not selling. They don't have the shelf life of whole grains.
If you're ordering the same style kits with any frequency, you definitely should buy grain in quantities suitable for a few batches. Even if you have to buy milled grain, you can use a food sealer to store it in vacuum sealed bags in the freezer and it'll last a long time. If you can mill your own, defiintely buy 5 to 10 lbs quantities of base malts and 1 to 2 lb quantities of the specialty and roasted malts you use.
In this day and age, you can order from anywhere. Surely there's a source for what you want, even if your normal supplier doesn't have it. If the larger sellers don't have what you want, just about any LHBS would put together a kit for you. The local store here in Austin mills kits on demand and could easily ship anywhere.
