I'm going to guess that most paypal sites are smaller dealers than the sites I've dealt with (Hobbylinc.com, hobbyheaven.com, towerhobbies.com) that don't offer paypal as an option. Of course, being in the US, I can typically find all the kits I need at the LHS. It's a shame that so many sites still don't use paypal.
Anyway, the only shop that comes to my mind is
Hometown Hobbies. Unfortunately, their online catalog is extremely limited and they don't have either the Viper or Mustang on their web page... in fact they only have 5 Revell kits on their site.
However, they do advertise the use of special orders through their distributor, so you can get it, you'll just have to drop them an email.