Getting a new keyboard for QMK?

[absence]

Are there any "mainstream" keyboards (i.e. just buy it at a store, no DIY or group buys) that can be flashed with QMK? Are there other easy ways to get started?

[killyou]

Although not QMK but some of the Vortex keyboards are fully programmable now thanks to their MPC tool. I think they will add support for the rest of the models later as it definitely was announced for their flagship Poker series.

http://www.vortexgear.tw/mpc/index.html

Maybe that's something that you would consider as Vortex is widely available in the stores.

[absence]

Thanks, it's a reasonable plan B. By the way, there seems to be work in progress on getting QMK running on Poker, but apparently it's not ready yet.

[killyou]

Thanks, it's a reasonable plan B. By the way, there seems to be work in progress on getting QMK running on Poker, but apparently it's not ready yet.

Readme on github mentions that Pok3r and CORE is already supported, interesting.

[Tactile]

You could pick up a compatible keyboard and install one of these controllers. Read more about them or ask questions here.

[absence]

At first glance, it seems like many of them are out of production. There's mention of an "unloved bastard" controller replacement for a current CM keyboard elsewhere, but the link is dead.

[ErgoMacros]

Web store, not in brick & mortar stores (as far as I know) but off the shelf ErgoDox EZ runs QMK and comes fully assembled.
(They also have a graphical keyboard layout designer that generate QMK .hex files for download.)

Another option would be a TMK USB to USB adapter. https://geekhack.org/index.php?topic=69169.0
TMK is not exactly QMK, but may have all the features you need. Available commercially, fits between "any" USB keyboard and makes it TMK programable.

And lastly there's a similar QMK converter here:
    https://github.com/qmk/qmk_firmware/tree/master/keyboards/converter/usb_usb
 but there are some disclaimers.

'Luck!