Usually pretty hard to track down that information unless you can find someone who has taken it apart and measured it.
Search youtube for teardowns is my advice. Actually Hardcore Overclocking / Gamer's Nexus commonly do tear downs and extremely detailed component review of GPUs. But you might also find some amateurs that have done it.
In general, order yourself some .5 and 1mm thermal pads. Generally up to 1.5mm is pretty common as a maximum thickness, so likely worst case you are using two layers. 2mm maybe, then you just double stack the 1mm. Anything more and you can consider thermal pad alternatives like thermal foams.