Depends on what you mean by LOUD. The loudest option is drags. You should be able to get a set for under $150 including some anti-reversion cones.
I have straight drags on my '91 Dyna, and it is one of the loudest bikes I have been around.
TQ
TQ is spot on, I have dual drag pipes (Drag Specialties) that were straight no baffle...VERY loud (window rattling load as gals at work said), no there were not any trees falling over, or women taking their kids off the streets, but did set off car alarms all along the streets near my house. Put some 12" baffles ($25 pr) to maintain a decent amount of decorum. :newsmile036:
I your case, I would look into replacing current baffles (you may need them if noise/EPA enforcement becomes a problem) and get Big City ones or similar like Hobbit suggested. There should be around $40 for pair on up...!