> Perhaps there is a similar alternative?
:-) ...Scala Sans was a rather original design without any direct predecessors when it came out, and while it may have spawned a few epigones, there still isn't really a typeface that's similar enough to be mistaken for it, except, to the untrained eye, Majoor's own Nexus and Seria.
Other faces from roughly the same category (neo-humanist sans) that are available at no charge include Lucida Sans, Adobe's Frutiger clone Myriad, their recent Source Sans (not really humanist though). And why not check out Microsoft's recent system fonts as well? They include Microsoft's own Frutiger clone Segoe, Corbel, and Candara, a proper neo-humanist sans with a decent italic.
A rather recent addition to the TeX Font Catalogue is Cabin. Two styles, four weights, true small caps. It may not be able to compete with Scala, particularly when it comes to the italics, but out of the fonts from that catalogue, it's as close as you can get.