http://www.bergquistcompany.com/
I have been using as a matter of preference materials from Bergquist for roughly 40 years, albeit in small quantities. Typical output-per-device is at/around 30+ watts. None have failed over dozens of amps at 4 - 20 pads each.. Some caveats:
a) There are a LOT of counterfeits out there. Make sure what you use is from a reliable source.
b) DO NOT reuse such pads. They will last 40+ years if left alone. But if re-used, they could have micro-tears or punctures from the removal and re-tightening.
c) DO NOT use them if the heat-sink or substrate is not smooth and flat.They are not grease that can be used in excess to fill such gaps.
d) DO NOT use them with grease/heat-sink compound or any similar material. They can be incompatible.
They are pretty simple, pretty basic, inexpensive items with a single significant virtue - they are not sloppy. So, compound on leads that interferes with soldering becomes a thing of the past (although that is easily controlled with care at application). And there is no measuring issue.
I have also used these:
http://www.talonix.com/shop/item.aspx?itemid=3639 - only because they were OEM. They can be reused. Expensive. For a number of years, AR used them on their US-origin amps and receivers.
With all that in mind, I keep a small stash of Bergquist materials, mica & compound materials and some mica sheeting (also cheap and easy to find at a Jewelry Findings, stove or lamp supply) for unusual situations and/or applications. Such as adjustable mica caps on vintage radios. NOTE: Mica does not like to be drilled - better to punch it. And to make a small diameter mica punch, get a piece of brass tubing with the correct ID, and file it sharp. Punch onto something fairly hard but with some give - I have a piece of maple flooring that I use. You will get perhaps 4 holes and need to resharpen, but you will not flake the mica as you would with a drill. Very sharp tin-snips or professional fabric shears do a nice job of cutting the stuff.
Horses for Courses.
Peter Wieck
Melrose Park, PA