if you had already started work with your builder and if your builder had indicated the supply and install of marble floor for you, you would need to talk to your builder on whether he can omit the item out from the contract should you decide to get from another supplier.
getting building materials from another supplier is not like buying an electrical appliance from another shop. your builder would most likely have factored in the cost of raw materials (marble tiles, tiling adhesive/mix), labour for laying of marble floor/wall, labour for polishing of marble floor/wall into his quotation to you. so by you switching to supply your own materials, your builder would still need to charge you for the labour and materials for install and polishing. on top of that, he could also charge you for profit in attendance. also since you are supplying the materials, if there's any damage during installation and you end up not having enough tiles, you will have to fork out additional money to buy more tiles since you are now the supplier and not the builder.
if you have a very big area for marble installation, supplying your own may save you some money. but do note that this amount can't be part of your construction loan (i'm assuming you have taken construction loan) and needs to be paid in cash to the marble supplier.
anyway, talk to your builder on this and work out the sums should you decide to supply your own marble tiles at this stage when contract had been signed.