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 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, 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.