Have a look
here
"If a laptop has an incompatible wireless card, determine whether it can be replaced, or find a USB wireless dongle which is compatible. The most supported USB dongles at the moment appear to be based on Realtek chips. See the urtwn(4) manpage for more information."
I do not use netbsd myself (instead I use openbsd on a vintage ibm thinkpad 600e). However I would have a look at the wikipedia page that lists all drivers for current supported wifi chipsets:
wikipedia open source wifi driver page
That said, usb wifi adapters with a chipset that is supported by the urtwn driver (available in netbsd, ported from openbsd driver) are usually both small and cheap. The only disadvantage of small/cheap wifi adapters is they have small integrated antennas which are inferior to the larger antennes that are built into notebooks (usually somewhere in the screen area).
I bought this really small one
EDUP EP-N8508GS (chipset Realtek 8188CUS) and it works stable under openbsd, though not very fast and only with mediocre reception; on the other hand it was cheap.
Edup-ep-8508 or
Edimax EW-7811Un are other examples based on the same chipset.
I also have a Linksys WUSB54G rev C, based on a
rum driver supported chipset, and it works really well. I guess you should be able to find such older usb wifi adapters by now on the second-hand market, or perhaps someone has one they no longer actively use anymore. The advantage of this particular one is its free firmware that can be distributed together with the operating system itself. This means this usb wifi adapter is also supported during installation of openbsd without the need for downloading additional drivers. However these usb wifi adapters are a lot bigger than the urtwn-based ones, and thus are more inconvenient for laptop use, are also carry an increased risk to damaging your usb port on your notebook, or the adapter itself.
So it all depends on how good is good enough for you...
Edit: I see now that
other people sometimes have problems with urtw / urtwn driver-chipsets combinations...