Maybe I misread the original question, but I thought the query was
for a less-expensive alternative to a sinusoidal stake. If so, I
believe you can get by for quite a while with a home-made version.
Where I teach, we only recently got a steel one. For years, we have
used a wooden one, then one made of Delrin (a resiliant white
plastic). You can start with a chunk of two-by-four or wider. Draw a
line down the center of the long axis, or, at least 1/3 way in from
the edge. Then drill a series of holes starting with a big one–
say, 2"-3". Then drill successive holes, each a bit smaller than the
previous, with maybe a half-inch between the finished holes. Then,
saw the wood in two along your original line. This gives you two
pieces of wood with semi-circular indentations of graduated sizes.
With a bit if work with a rasp and/or files to smooth the edges, you
have a very useful stake (well, two, actually). If you want it
sexier, drill along two parallel lines, alternating lines in drilling
the holes, so that the final result has the “sinusoidal” look.
I hope this is a clear word-picture. If not, maybe someone else can
help out. Good luck!