Keep an eye on GPS Warehouse, they get then but don't shout it from the roof-tops.
They only really come up when a model becomes obsolete and Garmin presumably clear their stock. All the 2610s, Quests and I think 2720s are all gone now. Next will probably come the Quest IIs and 2820s. I wouldn't bother with the former as apparently they're a bit pushed for processing power with slow map refresh rates because, I believe, of the compressed NT mapping. The 2820s will probably start priced close to the Zumo but will probably come down in price as stocks, and demand, dwindle.
The Garmin web site isn't much good as an indicator as to what's about to get the chop as they're still showing the Quest as current yet the 2720 has been moved to discontinued. That's on one site yet another only shows the Zumos...
If a Quest (the Mk1 version) takes your fancy then I should be selling mine next week.