Well, you can make it yourself by roasting some 2-row or maris otter, google that and get the procedure. Without brown malt, and with black patent added, it will be very different from Jamil's brown porter, but could be tasty.
Your best bet would be amber malt, as many can be as dark as brown malts, with similar character. Unfortunately, if your lHBS doesn't have brown malt, they probably don't have amber malt either.
You could definitely add a half pound of biscuit and a quarter pound of special roast for a nice character, but again, that's a different beer.
I can't be arsed to keep up this list of what's in the fermenters, but hey, check out the cool brewery I'm opening up!
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