Yes, Safety Devices is available in the states...
IIRC, Sube Sports advertises them in Excellence (may be another shop though, not sure)
And yes, they have cages/rollbars for pretty much every type of Porsche (and yes, their website just sucks

)
Make sure you get a "racing" version and not a "street" version
The "racing" cages/bars come with a certificate of FIA approval (and are build to meet/exceed those)
A "street" version does not...
A SD-rollbar can easily be expanded to a full cage later on if you want/need to
Joe is right, a rollbar/cage should never be bolted to the floor...
Safety Devices cages/bars mount to a reinforcement plate that is welded onto the longs
But I disagree with Joe that a bolt in cage is unsafe or crap...
Sure a custom made cage is better/nicer, but for some people that's just not an option