High Standard made a 10 ga bullpup 30 years ago or so and it's the only one I know. I haven't seen any aftermarket bullpup sets.
Try the maker.
ati
you can buy them all day for $250 and down
Try e-gunparts.com, local gun store, local gun shows, Shotgun News.
One can buy a transfer pump from any stock of gear pumps. Anywhere that motor parts or spare parts are sold, can also sell transfer pump. This item can also be found in any supermarket which has a department that stock car accessories.
Buy Shotgun. Buy Shotgun.
Buy parts for 410 Stevens model 67 pump shot gun.
I have a sears and robuck 20 ga. pump with adjustable choke on the end of the barrel
He can buy a shotgun, but, it is legally his until you turn 18
Probably not going to find an original stock in usable shape. You will have to buy one of the 95% inletted stocks available from companies like Boyd's and finish it yourself. sales@countrygunsmith.net
Baikal parts are available thru EAA Corp. Below is a link to their parts store. http://www.eaacorp.com/manuals-parts-diagrams.html
they buy it from the London stock exchange