ninja strength velcro?

need some ninja strength velcro to hold a sub, anyone have any ideas on Brand names or where I can get hold of ?

What's your situation?  i.e. vehicle, sub, enclosure, desired mounting location, etc.?   I'm guessing since you're thinking of velcro you're trying to mount a subwoofer somewhere and be able to easily move it.  There are many other ways to do this and they will probably be much more effective than using velcro.  Not to mention you're gonna have to use a whole BUNCH of velcro--regardless of how strong it is--for it to really be able to secure a sub properly.

Search google for "hook and loop" OR "hook & loop" and you'll get lots of results cuz velcro's a trademark name so companies can't use it.  In particular check out 3m's fasteners  cuz 3m makes some of the best adhesives and tapes available so I would bet their velcro-type fasteners are top notch as well.

In that case, a more secure alternative might be to get a couple eyehook screws to screw into the box, maybe one on each side of the box (except for the bottom).  Then run cords through the hooks.