Brown dog,
Ian is correct in suggesting contacting an injection molding company, for instance a place that makes plastic bottles. The raw material they use is in the form of plastic pellets. Another source is from hobby craft suppliers, since the little beads are the basis for dolls etc. They are very cheap, unless you have to buy them in very small quantities. Advantage is they do not hold water, they are light and easy to work with.
Although heavier I also use bags filled with budgie gravel, available in pet stores. Nicely screened so it is uniform, clean and easy to fill with also.
Now sneaking beanie-babies from your wife's collection is probably the last resort, but you got to do what you got to do /ubbthreads/images/graemlins/smile.gif