Electromyograph electrodes. Since any company that sells them only sell them to research institutions so they look rather hard to get hold of as an private individual/mad inventor. BUT WAIT!! Thanks to the internet and those intrepid sexual adventurers we have [1] e-stim, although I knew of its existence before, what i didn’t know/think of until I stumbled across it, they use the thing I’m looking for in reverse, [2] these!! and what’s more they even sell the conductive gell. Well that’s that sourcing problem sorted out, just need to find a photoplethysmometer/Geer gauge.

Oh and the volunteer to try it all on.