How to Get and Install Source SDK with L4D1/2 .
Download Source SDK or L4D1/2 Authoring Tools Download Source SDK or/and L4D1/2 Authoring Tools by going to Library --> Tools in Steams interface. If you want to create maps and custom content for HL2, HL2: EP1/2, Portal, CS:S, DoD:S, TF2 you need Source SKD .