Chris - they arent best. They are just better than flaky soft steel.
Here is list of places as of my recollection right now:
ExpeditionExchange.com - "Drendought" sliders - hideously expensive
Rovertym - With/without nerf bars - somewhat expensive
Safari Gard - usual sill mounted stuff. Gave birth to Rovertym's according to rumors - hideously expensive
DAP-inc - presumably made by Bearmach. CLaimed to be soft but cheapest.
Slickrockfabrication - claimed to be very good by users. Mid range price
Red rock rovers (Marc and Sam can correct me on name) - claimed to be very good by users (Sam can attest) , Mid range price
NorthWestParts - no comments yet, low range price
Roversnorth - no comments known, high range price to my recollection. Not sure of actual manufacturer
AtlanticBritish - terrain master sliders, rumored to be good, mid range (oddly enough) priced.
4x4ag.com - scorpion racing made, with/without nerf bars - mid range. Rumored to be very good by users.
TrekOutfitters - self made. Hideously priced but rumored to be good.
Aedofab - self made, questionable quality (heard both very good and very bad things of them), high range priced
Rockware - cant remember if they making sliders.
Rovertracks - can make you ones, should be reasonably priced, heard lots of good things of them.
All above seems to be sill mounted. All require drilling. Some has sandwitch design, some not.
Personally i like sliders to be frame mounted

Havent seen any yet.