Hi Geoff,
I may be in the market myself for some pistons and if I get to that point I
will be talking to Arias
http://www.ariaspistons.com/
also Venolia that does list Bristol in their catalogue
http://www.venolia.com/products.html . They are both in the USA and well
respected in the custom piston/ antique car / race car hobby.
I will be posting some pictures and questions in the next day or so.
Merry Christmas,
Dorien