you could try dulce de leche, which you can find ready-made in jars or make your own using sweetened condensed milk (directions can be found in my cheesecake brownie recipe recently posted). it's quite firm when cold, but softens up, but is not anywhere near as thin and runny as ice-cream topping caramel sauce. it's very economical if you make it yourself (you can use a generic store brand), and has a wonderful depth of flavour (it DOESN"T taste like butterscotch, which a lot of commericial caramel products tend to). you could definitely mix it up with buttercream. whatever caramel you use to mix in with your buttercream, you might want to pour it in a random stream, and then barely mix it up, leaving the buttercream very streaky so that you have chewy ribbons of caramel throughout the filling.