Seek and you shall find. gumtree seems to be the easiest place to find them, but because of the rarity of the 20" you just need to be patient. I got lucky when I found my soild deck because the seller had used Scott Bonner in the title instead of Scott Bonnar. Luckily for me I found it and got it for a good price of $130.
Personally I prefer the look of the twin rail, however I agree that the soild decks are the more durable of the two, particularly with regard to chassis flex with the larger width machine.
Arm yourself with knowledge such that when a 20" comes up you can identify if it is the machine for you. Nothing wrong with having a small toolkit handy to open the chain case to inspect the chains and sprockets for wear as well as the reel and bedknife for damage.
Winter is a buyers market so happy hunting
Simon
