There are several sources for synthetic furniture, most stock options are listed on the retail parts page in the FAQs, Choate Machine makes all of the factory synthetic stocks, are available direct from them, H&R, Midway and Brownells. Synthetic stocks and forends are also one size fits all as far as barrel contour and what frames they fit, the only difference will be variants in the different styles, they don't make any forends just for bull barrels, there's just less clearance for the heavier contoured barrels.
Tim