You can also get large batches of noname thermal paste or glue on eBay and they should be fine if you the LEDs aren't being pushed too hard.
I've got a tube of cheap thermal glue and it seems just fine for a Cree XM-L @2.8A, the LED MCPCB and heatsink both heated up relatively evenly. I don't have a IR camera or thermometer to check actual temperatures though, and the MCPCB might not be the best either.
The thermal paste is only meant for filling in gaps between the two metal layers, so when modding the light, use as little thermal goop as possible and make sure the layer is thin and even.
Thermal paste doesn't transfter heat as well as the meta to metal layer (
even some tooth paste work decently as a thermal compound from what I've read).
If you are using glue, keep applying pressure until the glue sets.