You probably know that the period correct outlet looks like two RCA style speaker plugs as found on the back of a stereo amplifier. As in
But since these work well with only my period correct brass “trouble light”
and Happy Jack’s hilarious portable tea kettle (which is hands-down the absolute best TDr accessory I have ever, or will ever, run over in BillinPart’s TD race car), the standard USA ciggy lighter makes more practical sense.
Grainger has this one for $10.
JBugs has a VW one here for $20.
Here’s an underdash one —two actually–for $5. I put mine under the dash on both cars. Easy, convenient for plugging in the iphone charger and no worries about how inappropriately “modern” they look.