If you're after 2 then the cheapest good wheels might be the wooden trixie wheels
https://www.zooplus.co.uk/shop/small...8aAgDNEALw_wcB
They're generally pretty good, I use them for my robo and like them. You're after the 8 inch/21cm one. It'll be bigger than what they're using probably, but it's a much better size for them.