Local muffler shop says there are 3 options from Catco, Walker and Magnaflow, perhaps more. $423 installed. Another shop is $350 installed.
Alfa Parts had no parts available but directed me to local resources.
I have no experience w/the others, but Magnaflow has been very good for me. Amazon sells 'em.
1982 Spider Veloce
1999 BMW R1100S