Hey Hadley - I've ordered various made-for-modelling ones over the years with mixed success, but the ones I get the most use out of are for nail care, from the local chemist. Do look carefully first though, you have to be discriminating in selection because many are too heavy a grit.
To be honest I've never been very enthusiastic about the utility of most sanding sticks, because they are generally foam backed, which has a tendency to round off edges and generally produce very imprecise sanding.
I have a number of wooden blanks of various sizes, to which I double sided tape various grits of sandpaper (400 up to 1500).
The only sanding sticks I really like are the flory ones. I generally like the green/white polishers for finishing, and also find the fine and the medium skinny sticks are also useful. I will sometimes use a sponge backed sanding stick on rounded surfaces.
Other than those, these days I do 90% of my sanding with my home made variety as the flat surfaces ensure more precise sanding.