I have used a paintbrush before, once I lost mine for a long while and so I found a paint brush end that fit in the slot and cut it to where it was smaller and used that until I finally found the place I had put it. (Turns out I had put it on my black keyboard and didn't notice it there, because it was black on black, until I had to hit an F key up top...)
If you're looking for replacements, just pop onto Amazon and get some third-party ones. I bought these for ?3.49 and haven't had any issues. In fact, they fit more snugly that the official ones, which is a plus since the plastic always ends up wearing down over time anyway.
One option that I didn't see before is this: 99p with free delivery for five styluses! That's the option I'd recommnd if you're ordering from Amazon UK - no doubt there will be similar listings for other countries.
When I played Cooking Mama 2 on my original DS I had to use a mechanical pencil, but now that I have a 2DS I just use my nail whenever I don't feel like pulling the stylus out for something.