Sorry to necro, but I saw that @shanaqui mentioned in this thread shanaqui's Beeminder Journal that they were interested in getting a physical button.
Whilst that list linked by dreev is still mostly up-to-date (though AFAIK Amazon have stopped selling Dash buttons), I had a couple of things to add for those looking at physical buttons.
They might not offer them anymore, but the ‘single purpose’ Flic buttons (eg LIFX) that they used to sell for cheaper work just the same for the purposes of triggering eg Tasker. Also, now that they have released 2nd gen Flic, you can get a 4 pack of 1st gen Flics for $60, so they’re much cheaper than they used to be.