I was doing some research on 3rd rail heater splicing kits and had to find step-down uninsulated butt splices. The only place I found them was www.terminalsupplyco.com.
Their HQ is in Troy Mi. and they have 6 Satellite locations. I downloaded their catalog pdfs(once I got DSL) and couldn't believe all of the odd ball stuff that they have. I'm not saying that they have what you need but they are well worth a look. I would check their terminal & maintenance catalog sections first. Good luck with your hunt.