Where can I find cheaper RAM for my PC build?

Check the used market first. Vinted and Leboncoin are worth a look in France, especially if local collection isn't practical. Compare the total with shipping and ask the seller for the exact model before buying.

Make sure it's the right DDR generation and desktop size for your motherboard. Cheap laptop RAM isn't usually a direct fit in a desktop.

What motherboard and CPU do you have? The cheapest compatible RAM depends on whether the board takes DDR4 or DDR5. Once you know that, compare 16 GB kits across retailers as well as used listings. Don't buy based on capacity alone.

One 16 GB stick could get the machine running for less upfront than a larger kit, with another stick added later. There can be a performance tradeoff versus a matching pair, and finding the same model later isn't guaranteed.

Laptop memory can look like a bargain, but factor in the cost of an adapter and check compatibility carefully. It may not end up cheaper than a used desktop stick, and it won't fit a desktop slot on its own.