Just wanted to chime in that I've been using Instead for years. It's available in most grocery and drug stores. Economical, too, as the cups are actually reusable (if washed well and promptly).

Two warnings:
1) it takes a day or two to get used to, and you'll have to "explore" for comfortable placement... so pick a trial period day that you don't plan riding and let your body adjust.

2) b/c you empty and wash the device (as opposed to discard and replace, as with a tampon), make sure to scout public restrooms with a sink nearby the toilet.

Besides athletic convenience and economical benefits, there are also huge environmental and health benefits to such products. They are much safer for the female body, and it's kind of empowering to get over the 'squeamish' factor that's been engrained in us.

Just my two cents!