Brown malt IS commercially available. It won't be exactly the same as historical but it might be the best we can do.
Actually when brewing historical I simulate wood-drying by adding around 5-10% Bamburg smoked malt for part of the base malt, gives it a little something.
I've also toasted my own malt many many times to produce amber or brown malt. It will usually lose its diastatic power so don't rely on it for more than say 30% of the grist. But you can do this easily in your own oven just by spreading out a couple pounds or whatever on a baking sheet in a 375-ish F (190 C) oven for 20-45 minutes until quite toasty, then cool, grind, and brew as normal. Deeply toasted flavors, a little goes a long way actually, probably don't want to use more than 10-20% of the total grist.