Things that aren't made any more won't be available and plentiful forever, just like Model Ms are largely gone from thrift stores nowadays. If you're after certain things that only old boards offer (let's say you want two-tone beige doubleshots, complicated ALPS, NMB Space Invaders oder Montereys), it's probably not a bad idea to stock up while you can. There is no reason for prices to go down.
It's probably not a smart idea to stock up on Model Ms unless you're a collector, since Unicomps make a good functional equivalent that can be bought new at a reasonable price.