Usually what I do is use stuff from my bitz box (once ive built a model all the spares go in there) and glue. I HAVE heard people talk about some green stuff that is good for modelling - I personally have never even seen it but a guy I see around the forums ( @Rikard) is both a master at sculpting and the go-to guy for any queries about modelling or converting. Try asking him
Use the left overs of other kits. The extra heads, feathers or weapons are perfect for cool simple conversions. Once you get bored of just swapping the heads (which will happen 100%) you can go next level and get some green stuff for minor texture sculpting and you´ll see how easy conversions are. I think long ago GW sold metal lizardmen glyphs, maybe you can find a set on ebay