For an extension you need a card that uses standard RP-SMA connectors. Then you can buy any generic RP-SMA extender to move the antennae(s) anywhere you like.
As for which device…
| Standard | Freq |
|---|---|
| Wi-Fi 6 (802.11ax) | 2.4/5 GHz |
| Wi-Fi 5 (802.11ac) | 5 GHz |
| Wi-Fi 4 (802.11n) | 2.4/5 GHz |
You’re looking for AX or AC wireless.
Here’s the tricky part… beyond wireless N there’s no more FOSS firmware which means no more guarantees of working out of the box. In many distros like Debian and Fedora you’ll need to enable a non-free repo to be able to install the firmware
I did a deep dive into AX/AC/AD Linux support and here’s what I uncovered:
https://forum.tuxdigital.com/t/community-list-of-wireless-devices-with-foss-firmware/3704/5
It’s a semi-infinite rabbit hole so I had to call it quits but the idea is you find the supported firmware (all found), then the chipsets that use that firmware, then the devices that use those chipsets and you have purchase options.
While deep diving the only devices I found that i’d be comfortable buying (assuming I didn’t mind non-free) are both AC, see below:
Linux Support: Linux support for MT7612U based products - ALFA Network Docs
Linux Support: Linux support for MT7610U based products - ALFA Network Docs
…or any device using the MT7612U or MT7610U chipsets.
** Edit: Thanks to gemmakaru I found Intel’s list of Linux compatible devices which include three Wifi 6 models: Linux* Support for Intel® Wireless Adapters **
Alternatively if you’d be happy with Wireless-N there’s lots of “works out of the box” FOSS options here.