Why need pcie 3? Pcie 4 ssd can use on pcie 3 system. Unless u try find tlc then that's different requirement.

Many reputable online store. All IT, EIT, Mono digital, brightstar. Shopee Mall or LazMall also have Sandisk official store I think. If in doubt try search in lowyat for review. Just make sure it's the real store, not a fake store using the real store image. For example a random .my name store using EIT store image in their product. Some store also have sus listing, like seagate agent store selling wd drive.

Now sn570 or sn580 are already discontinued

Pcie 4 disk is backwards compatible with pcie 3 board slot, just that speed will be little bit lower in benchmark, however will have no effect at all in daily usage

Now if want look for WD/sandisk at least Blue SN5000, 2tb and below are still TLC flash