Web sites for System 3, Gougeon Bros (west System) and MAS are good places to start. They have many how to tips and either list suppliers or you can order direct. Look at Nick's building pages for web addresses and other sources.
My personal favorite trick for applying fillets is to put the gunk in a plastic freezer bag, force it all in a corner, snip a small piece of the bag corner and then squeeze it on, like frosting a cake. Then you smooth it out nice.
Have fun