Here's a link to the part: http://www.ereplacementparts.com/poulan-pp4218av-type-gas-chainsaw-parts-c-16962_17147_17808.html
Good luck.
I agree totally with Dan. This is such a frustrating problem with a totally unacceptable solution. I found the part on the Web, but it was listed as Back Ordered with no delivery date provided by the manufacturer. So, we're all stuck! For all the sales of this chain saw the manufacturer should simply come up with a replacement cap for the marketplace and stop using the current version. My goodness, the frustration and then the stonewall have become complete deal-breakers for me purchasing other Poulan branded products. I'm done.