I've had several WONDERFUL experiences with e-weddingbands and highly recommend them, especially for bands or simpler designs! They're incredibly responsive and helpful, and they work very quickly, and they're committed to finding exactly what you want - even if it isn't stock they can probably source/make it for you
I had a good experience with jumpey2003 on eBay. It was a few years ago but the item was good and very inexpensive.
I purchased a 1mm yellow gold band to work as a spacer with my then AVC and eternity band.
I am looking for a 2 mm band. In fact, it's not really a spacer. I lost both my wedding bands, can't find em. So bummed. So I wanted to get a very low band. I found this one on e-weddingbands.
I would go with the same metals. Over time the 18k white gold will look whiter than the platinum e-ring which will look grayer as it develops patina.
I prefer their comfort fit bands - they're like butter sliding on and off lolol... Hmmm that looks grosser in text than it sounded in my head. They should have a few options in the 2mm plat...
Definitely get the same metals. For a legit spacer, I went with Jeweler on the Roof and it was, to me, worth the money. The service was great. But for a 2mm you could go with e-wedding bands, or get one made to order from a vendor knowing the height you need. Mine is from Stuller, and is not low, but I know they probably have a low-dome option.