There are no 802.11ac radios that are as well-supported as the ath9k 802.11n radios. All 802.11ac radios require closed-source firmware. Qualcomm-Atheros, MediaTek, and Marvell 802.11ac platforms are getting the most open-source attention. QCA chips are really popular and the drivers are pretty mature, MediaTek's firmware is flexible enough to make it one of the more hacker-friendly platforms, and Marvell's radios are often paired with their really powerful (by router standards) processors.

Unless you really need the throughput of 802.11ac or a more powerful processor to keep up with faster WAN speeds, ath9k 802.11n devices are still a pretty good choice.

I'm been replacing ASUS RT-N16s with RT-AC56Rs. $50 refurbs in-store at Microcenter, $75 new from Walmart.com, well-supported by all the firmwares that have ARM builds.

If you can't buy them that cheap the TP-Link Archer C7 has been a better value... well, up until now.

According to the OpenWRT wiki:

http://wiki.openwrt.org/toh/asus/rt-ac56u

The BCM4352 (AC/AN chip) is not supported, which is part of the reason to get one of these routers. Broadcom router chips are usually MIPS based, and also heavy on the binary blobs (which may allow for dd-wrt support).

Edit: In fact, I can't find any image that supports this BCM4352 on dd-wrt either:

http://dd-wrt.com/wiki/index.php/Supported_Devices

Not exceptionally cheap, but:

https://omnia.turris.cz/en/

I'd really like to know this too. Been sitting on my wrt54g for far too long and starting to feel the need for speed.