In my experience, and old USB wi-fi adapter should work. I've bought several off eBat for ~£5, and never had any problems. Unless your router has specific requirements, pretty much anything should work. The only thing to look out for is that, if your router supports b/g/n wi-fi, you should ensure the dongle does too. Some of the very cheap dongles only support the slower b and g wi-fi.
OnePlus 6 review: Hands-on