Free 2nd visors with some lids purchased here:-
http://www.thevisorshop.com/acatalog/index.html J&S are selling Arai corsairs with £100 off:-
http://www.jsaccessories.co.uk/hermes/shop/13,1,100 There's also a few shops on fleabay selling lids at reasonable prices.
As for advice, get the one that fits the best.