After placing some sign blocks you can change what these signs show by clicking on the bucket symbol (or pressing F5 if I remember correctly). Now, the blocks with changeable signs are highlighted. Click such a block and a list of several signs appears (arrows, warning, adverts). Or are you looking...

I just created my own track but I need some signs (arrows, warning ++) but I cant find the signs anywhere, hope someone can tell me where to find them.