I had the exact same kit as you and I was trying to get a 16 x 16 floor tom but I missed one on ebay for $340 then another one sold for $500 the next week. They were both used and had their share of scratches and dents. Good luck.
Any pearl dealer will be able to custom order it for you. Rupps just has it set up nicely on their site and they have free shipping which is nice. I got a quote from Guitar Center about 6 months ago and they wanted between $375 and $415 depending on the size and mounting type plus shipping. And it was going to take 6 months to manufacture and ship from Taiwan. I couldn't justify spending that much on one drum for a kit that I wasn't crazy about. I ended up selling my session customs for $500 and finding a Gretsch Renown with the sizes I wanted for $675.