Ebay is a great place to buy second hand miniatures.
Miniatures by post is a good place to buy new warhammer it has a 20% discount on most games workshop products.
Unit fillers can help to stretch the models you have further, for example you could have a 40x40 base in a horrors unit with a chaos spawn in it taking up four places.
horrors can be made out of lots of different things I saw somebody who used night goblins and hollowed out the face and removed the hands then painted the cloaks pink and blue. and somebody else used ghouls without heads and painted the stump where the head should be as an eye.
Plague bearers can be made out of lots of different things too I saw somebody who used kroot bodies with ghoul heads and a few horns.
Furies can be made out of lots of things just stick wings on ghouls or bloodletters.
