I got my grids and correx from C&C Guinea Pig Cages - Home - c and c guinea pig cages - you could perhaps get the pieces cheaper if you bought them separately but getting them from this website (correx pre-scored etc) is possibly more convenient (assuming you are in the UK, other members may know of websites in the US or elsewhere).
I got 36 grids from Amazon for 39.99, by far the cheapest I found. I have 2 boars and have set up a sleeping area, loft area and run. Might be worth ordering extra connectors.
My 2 were introduced to each other in there and stayed in there. We have a hay loft only the dominant one has figured how to get into so he chills out in there. Mine is an odd shape as it goes under the stairs too but it's so much better than a cage
I purchased two lots of the white, 9cube storage units, (two different occasions) and this has made me 2, 2x4 areas for my two sow pairs. The elevation obviously used more grids than I found it was on the floor, but I needed this in order to fit these sizes in.
I purchased "floor protection" sheet from wickes, for £3 and this made both bases for my c&cs.
check here for different places to get grids and videos of how to build your cage. watch the clips first before purchase to know the number of grids to get and size of correx/coroplast. Guinea Pig Cage Options To Choose From - SFF