I have four DSL modems from various providers. Most of them are interchangable. There is no registration procedure, but on the newer modems I did have a big pain in disabling the PPPoE built into the modem so I could let my router do it instead. I'm not entirely sure where you would buy one, though. They don't sell them at Best Buy - those are CABLE modems.

Just curious - what was your old modem model #?