A suitable alternative maybe something like Douglas Fir. As this is used in the building trade, it maybe available from your local builders/timber merchant.
If you get stuck, then check out this site under merchants. Should give you something in your area for specialist wood suppliers.
Not sure how good your DIY skills, but my tiller is laminated in three parts. I have no doubt this was done in order to prevent any inclination to warping.
I am sure one piece will be ok, providing it is thoroughly dry before you start . Any slight warp, if present, can be removed in the shaping process.