1.75. If none local, can you order?
1.5mm won't be tall enough so it won't make contact. This will cause overheating. 2.0 might work, but they will be too tall and you'll have issues getting the heatsink on tight enough. Thickness is also only a part of the issue. You need to worry about their heat transfer rates as well. Not all pads can carry the same amount of heat well.
Were the factory pads soft, medium, or firm hardness?
Changing pads on a gpu can be a crapshoot if someone hasn't already done it with a particular brand or model.