Available as kits from https://candcguineapigcages.co.uk/
or you can also find supplies & build your own via Amazon or Ebay & correx sheets are available via Ebay or any sign makers. You'll most likely want to also cable tie each grid to secure them as the connectors can be a little unsecure sometimes.
IMPORTANT: grids should have 9 squares horizontal & vertically. If the grids have less thatn 9 squares then the squares will be big enough or smaller guinea pigs to escape or potentailly get stuck & hurt themselves.