I’m looking for a cheap GPS tracker that can fit in a pocket allowing me to find, or track the wearer if necessary.

I know it sounds creepy, but it’s not…

There are a bunch of things on eBay for less than a tenner. Any tips or recommendations?

There’s almost certainly a very good reason why they’re less than a tenner…
There are things like Tile, but I’m not sure if that does what you want.
Thing is, in order for them to actually work, they have to be able to send location data, which means somehow utilising the telecommunications networks, which usually means an annual subscription.
http://pocketfinder.com
https://www.trackershop-uk.com/product-category/personal-gps-trackers/
https://www.personalgpstrackers.co.uk/gps-tracker-location-coordinates
Tile is around £30, there are a bunch of similar things here: https://www.amazon.co.uk/tracker-gps-location-finder/s?ie=UTF8&page=1&rh=i%3Aaps%2Ck%3Atracker%20gps%20location%20finder
They work by using BT triangulation through nearby BT enabled devices, IIRC.

Some of them take a SIM. You text it, and the tracker sends back its GPS location.

Would an old phone work?

Probably, with a tracking app installed.

Any old iPhone with “Find My iPhone” turned on?

Yes, but I don’t have an old iPhone. I’m going to buy a cheap GPS tracker and see how well it works.

You could pick up an iPhone 4 for about £35 on eBay? Probably lots of older Android models that’d do the job too.

I got one of those cheap trackers a few years ago. (I was going to track team mates on a 24 hr race).
It works pretty well. As said above, you need a sim loaded with some text credits. You then call it from your phone and it returns a text with it’s location. You can also set it up to send you a message if it goes outside an area you set up. Obviously it needs a mobile signal to work.