There are a number of them in the Twin Cities. For starters, there's Plymouth Bible Chapel in Plymouth, Believers Bible Chapel in Coon Rapids, Northeast Gospel Chapel in Mpls, and Longfellow Gospel Hall in Mpls.
Feel free to just show up. Non-believing visitors are welcome to attend and observe.
If you go, I'd be interested in hearing from you afterward.
Thanks. I'll try to get to one before Christmas. I was researching this myself, and was surprised to discover that Garrison Keillor was raised in a Plymouth Brethren church in Anoka.
It's a denomination I have not visited, so it's interesting. I've rarely found any church that doesn't welcome visitors. It has happened a couple of times, but very rarely.
Still, I usually contact the church before attending, just to find out what the best time would be and to let them know that I'll be there as a visitor. It helps prevent any awkwardness that might occur (I speak from experience.)