It's a fallacy to equate low brass with low pressure - it may be or it may have a high peak and fall off or again in this era of solid basewads that are their own reinforcement it may peak quite high and long and still be low brass. Making your own is reasonably straightforward. Have you had chamber length verified?
Powder Inc
http://www.powderinc.com/products.htm in Arkansas catalogs blackpowder reloads in modern hulls at about $15 a box - 12 Gauge BLACK POWDER Shotgun Shells loaded into once-fired Winchester AA hulls, HARD SHOT and TRADITIONAL BLACKPOWDER. - there will be shipping and storage issues with black powder. Collector or quasi collector 65mm shells out of England seem to run about $3.50 each. Corbon catalogs 12 ga. 1.5 oz of #8 for the Cowboy Action trade in blackpowder and the brand is very widely distributed. Likely enough there are others in the Cowboy market - try local cowboy matches and ask.