It's actually a relatively easy thing to make yourself, with plenty of options.
Solid wood, if your tiller is not 'curved' simply get a nice piece of hardwood and trim it to shape, may need to glue two or three planks together for thickness..
Consider making a 'straight' tiller out of Aluminum channel, and fashion a more user friendly place to hold on to it, or bolt 'u' handles on either side (extends your reach slightly) - and no more varnishing!
Laminate your own to the curve you need, jigs are quite easy to make with some angle iron and a heavy piece of plywood, then choose a couple of different coloured woods, cut them into appropriate thicknesses and glue/clamp into the jig. Shape, sand and finish to suit.
All that said I expect you'll find standard tillers at various marine suppliers if you look around.