If you are looking top replace a single roof tile or a small amount of roof tiles you may be able to get a match from a salvage yard, this is especially useful particularly for older style tiles that may be out of commission. Missing roof tiles, even very few, can quickly escalate into a much bigger problem because a missing tile creates a space where water can penetrate as well as allowing wind to get into the roof space and possibly blow off more tiles. Always make necessary repairs as quickly as possible. If you are unsure always ask a professional and trained roofer.