The following site has them at a reasonable price plus if you click on Color/Pattern choices at the bottom of the page, you can see exactly how to measure your girl to get the correct size: http://nu-nee.tripod.com/bb.htm
Whatever you decide to use try to remember that the likelihood they last more than a day is low unless you are incredibly vigilant. Personally, I gave up on the panties. The bleeding was never very much & Ella kept herself incredibly clean so I rarely saw or had blood spots to clean up. The worst were my sheets (yes, my spoiled B sleeps under the covers at my feet). I tried keeping panties on her but by morning they were shredded as well as sanitary napkin bits everywhere. It was cheaper just to change my sheets daily.
