Hi, I'm looking for an E29 lamp screw base. If anyone has any that they're willing to sell, or knows of a source, please let me know! Thank you.

I think one of the best places to look for E29 admedium base lampholders is to search around for inspection blacklight fixtures on eBay such as ones made by Magnaflux as a good number of them seem to be designed for the E29 base 100W H44 PAR38 mercury vapor lamps.

Here are some examples I found:

https://ebay.io/m/FggGPi

https://ebay.io/m/sDqcWi

Unfortunately, a good number of these fixtures are very pricey as they are largely intended for specialized applications like non-destructive testing of materials.