I personally wouldn't replace the original ones, just clean them as much as possible. You can use a bit of acetone and an old disposable napkin to gently clean as much as possible. Worked very well for me when I had sticky rubber problem with one of my controllers. Probably won't fix the yellowed parts, though, but at least they'll be clean.
However, if you really want to replace them, you have two options: 1) ordering third party ones from sites like ebay or local game shops (quality of sticks vary, they aren't all that great compared to original ones), 2) see if you can find used / cheap / broken nunchuks (as far as I remember, they use the same sticks and some of them may come with sticks in pretty good condition).