After chewing on some grains side-by-side, I think you can get fairly close to the flavor of brown malt by mixing special roast with a little pale chocolate. Of course both of those are sometimes hard to find at your LHBS as well. What percentage of your recipe is brown malt?
Brown malt is fairly unique. I use it all the time. I find there's no real comparison between the real deal and homemade or blends of other malts.
If your LHBS doesn't have it, they should be able to call their supplier and get some for you quickly. Otherwise, Northern Brewer has it and can get it to you in a couple of days.