A black cami in polyester (not silk) satin or equivalent to wear under lower-cut tops and sheer blouses. I'd prefer a bra-friendly tank style, but spaghetti straps are OK. No embellishments. Very thin. Not too low cut or it defeats the purpose.
Doesn't need to be online - if you see something in a chain store let me hear it.
This is the closest I've come but I'm not super excited by it, and it's way overpriced. Something like this should be available for less than 20. I'd be open to hemming a cheap slip/nightie too.