Elisha,
As 2Cdo said, those of is who earned the ballcap are unlikely to part with one. Which is why I doubt very much that you will find them for sale on eBay. There weren't all that many made in the first place, and I don't know of any surplus stocks remaining for sale. I'm at 3 VP, and there are none for sale in the unit canteen.
You best bet for a decent replacement will likely be the cap sold by Joe Drouin Enterprises / The Airborne Kit-Shop. He stocks a limited selection of Op Apollo/Rakkasans ball-caps, T-shirts and coins. The cap isn't exactly the same as the tour original, but it is as close as you are likely to find. Here are the links:
Home Page (with ordering instructions): http://www.joedrouin.com/
Rakkasans Merchandise: http://www.joedrouin.com/rakkasans.htm
If you scroll down to the bottom of the merchandise page you will see the tan ballcap. The only difference from the cap you are trying to replace is that it says "Rakkasan" on the back, rather than "Task Force Rakkasan Kandahar Afghanistan". The flags and Torri symbol are the same as the original.
You can order from Joe Drouin with absolute confidence. I have ordered via e-mail in the past and have always received the goods in a very efficient manner. Not to mention the fact that Joe and his Wife are a genuine pleasure to deal with.
Cheers,
Mark C