For a simple, straight, tiller the cheapest way may well be to make one yourself, and it is easy enough to make. Dimensions are optional, but 4 feet seems a fairly popular choice of length. I have just measured up the slightly longer one of two which I have at home; that one is four feet, while the other is perhaps a couple of inches shorter.
I attach two photos of the tiller which Ben Dallimore made for me a dozen years ago, for A Capella. I don’t have a photo of Douglas Boatyard’s work, but they are a general boatbuilder, and when I was visiting a few years ago and was in their chandlery I noticed and admired a nice laminated tiller which they had on the shelf. I casually asked whether it was one of Ben Dallimore’s, and was told no; it was one which they had made for a client, and he was about to collect it.