Driver, welcome to the group. A lot of us had one when we were kids, so we kinda forget some people are new to the brand. I myself just got back into it after 30 plus years.
The Portland convention is the Vintage Motor Bike Club, in Portland, Indiana. They've got a good group, but a poor website. I'd like to find a better chat group for those guys myself.
These bikes are a lot of fun as long as you don't expect jack rabbit starts. Blinding speed is not one of their virtues. It appears there's only one source for parts (besides ebay), but the guy is a swell fella who is very helpful.
Don't know if your into long distance procurements, but you might want to consider contacting Adam or Leslie Wollard in Bradenton, Florida. Their father (who past away last spring) had an extensive collection of Simplex's they are liquidating the collection. Otherwise, keep your eyes open on ebay & craigslist (I found mine on ebay just a few weeks ago).
Lots-o-luck,
Mike Sal