Hi Jack, the 'Bullseye' finders are extremely easy to make yourself. Just get some cheap acrylic, some suppliers even have offcuts for free. I leave mine as a square sheet as it fits down the side of my toolbox.
I drill a hole in the centre, of the right diameter to take a bradawl point, then use a set of dividers to scribe circles at 5mm spacing. Then drill a 1.5mm hole on each of the circles which will allow you to insert a pencil point. I drill those in a spiral pattern which reduces the chances of the sheet splitting along the line of holes.
They are great for getting the best circle out of irregular shaped logs etc. not just finding the centre of circles.
If you want, I can post a picture of mine, but the earlier posts, of 'commercial' ones are probably sufficient.
One thing I should point out, my business has nothing to do with the American company of the same name which sells the Bullzeye centre finder, that was just coincidence many years ago.