I'll second the recommendation to check eBay, auctions, and maybe Craigslist. I think you're most likely to find what you want in an older used halter. I bought a fairly cheap western show halter a couple of years ago, and everything I saw new at that time was all dripping in silver, flashy buckles, etc. Recently, just scanning tack catalogs, not seriously looking, almost everything new seems to be of the "congress cut" style as well. I do think an older used halter will be most likely to meet your criteria.
It's tough to find plainer western stuff - I'm not at all a "glitzy" girl, and I had a heck of a time finding western show tack and clothing that I liked. I'll keep my eyes peeled, and I'll be sure to post back if I run across anything that looks like it'll fit the bill! :)
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown