you just need an aran-weight wool. Cashmerino is very soft, but if you can find something that knits at 18st to 10cm, you can substitute. Sirdar supersoft aran would work (if it is for kids) although it's 100% acrylic (not for me) or Hayfield (20% wool) (have a look here www.ethknits.co.uk/frames/frame.htm or if you want wool, Drops Alaska is only £2 a ball www.scandinavianknittingdesign.eu/index1.html. Others have said good things about New Lanark but it is not, to be fair, supersoft.
Janette's rare yarns are worth a look: egsome here and Cucumber patch also have specials on discontinued wool - might be worth a look.
And Ebay is also worth a try for an aran-weight wool.