This is what I’d normally recommend:
But it’s out of stock alongside a lot of other Pi accessories in the shortage.
When searching for a replacement, I found that the connector on the CM4 board is MHF-1, which is apparently compatible with the u.FL connector featuring on our other antennas, such as the parts below:
Keep in mind, though that this antenna will likely only work well in the 2.4GHz band, which generally has more range but less throughput than the 5GHz band used in WiFi these days.
Perhaps you could look at stand-alone access points like those from Unifi? They tend to be fairly affordable for the performance they offer. Not nearly as cool as getting your Pi to act as one, but maybe a bit more practical.
Let us know if you need any more help picking parts.