There is no source of premade collars and cuffs of which I'm aware. Plus, you would need to take it to a competent tailor, preferably one who is a shirtmaker, unless you are yourself sufficiently skilled to do this. They would then have to detach the existing collars and cuffs, cut new ones, and attach them. If they made the original shirt, this is a much simpler process, and not all that difficult for them to do. If not, more so. But in either case I'd be surprised if the cost is less than a new shirt unless the shirt at issue is fairly costly.