I have one of those conversion kits as well and I recall getting mine for a bargain (even with the Yahoo! Japan commission and fees paid to either Rinkya or Okshon (which are auction proxies in Japan), I don't recall which exactly.
There is one currently offered on Yahoo! Auction Japan.
The name of the Company that manufactures these (and is a regular at Wonder Festival) is called "go-ki-gen-sei-mitsu" and spelled as follows in Japanese (or I should say Chinese characters): 御機嫌精密
Graham, if you read this, you might want to upgrade this information on the WonderFest article on the main page.
Below is a link that can be copied onto your browser to locate it on Yahoo! Auction Japan.
http://search.auctions.yahoo.co.jp/jp/sear...=0jp&acc=jp
By the way, the instructions allow you to use either the VF-22 or the YF-21 to do these conversions. Be warned, however, there is a lot of chopping and slicing up of the Hasegawa kits (including parts that are not necessarily prone to chopping and slicing).