What do you have now? If it's vinyl, you can just replace with more vinyl. It'll darken too, but it's cheap, quick, and acceptable to most surveyors.
If you want to get a little more permanent, you can put in polycarbonate tubing like I did. Removed the hose barbs for the vinyl and put in regular brass compression fittings from the hardware store. You can search for polycarbonate tubing on the web, or possibly find it at a local source that supplies gas stations and petroleum businesses. You can also find it at http://www.mcmaster.com/
*Just search for "polycarbonate tubing". McMaster also has some interesting hardware and valves for fuel systems, including a self-closing valve in case of fire. Of course, you should keep those valves closed all the time except when you're actually checking levels.