absorption foam,
auralex might be the best known, expensive though. You have some companies making more or less the same stuff for less. DAP for instance, American comanpy. Still expensive, but a bit cheaper then auralex.
Auralex Acoustics | Total Sound Control
Sound isolation is a very time consuming, costly job. That's probably not what you intended. You probably talked about accoustic treatment of your room to deal with reflection and such.
When you actually meant getting a room not to leak any sound, like being able to play loud music without your neighbours complaining, the most important is that every little opening needs to be closed. You even need to go as far as filling your electric wall sockets and stuff like that. Then buying specialized sound proofed vent systems and such.
You basically need to aim for making the room able to hold water
and the walls need to absorb and diffuse sound as much as possible.
If you need to practice a loud instrument a lot, you usually get the furthest by getting something like this:
Which will suck because of the reflection and needs treatment
These guys among others supply you with accoustic boxes to place in rooms
STUDIOBOX Acoustic Studio, Isolation Booth, Sound Booths
Just google for tips but make sure you get your terms right:
Sound proofing vs accoustic treatment intended for studio usage