DaMadFiddler found a nice place that sells Broadband Adapters for Dreamcast:

eStarland has them in stock:

http://www.estarland.com/SegaDreamca...uct.29024.html

They're pretty pricey ($160), but I've ordered from them several times in the past and always had good luck. Came across this today while looking for a few other things, and figured I'd post it here in case anyone was interested.