I love Kona coffee, obviously for the price its not worth it (especially not to add to a beer....) but if you ever do get to try it, its brilliant.
Its not very bitter and is quite nutty and almost a bit chocolatey in flavour.
As a replacement, I would probably try to find a lighter roasted Kenyan or Ethiopian coffee with a quarter of coffee from central america. Try to find whats on offer and go with that.
To be honest, unless you are a real coffee connoisseur anyway, I think its unlikely you will get the subtleties from the coffee in the beer anyway.
Sumatran coffee is quite unique in its general flavours... although like anything these flavours may not show through so much unless compared directly with other coffees. its meant to be quite earthy and herbaceous with less acidity than others.
Not sure any branded coffee in the supermarket is going to give you similar flavours to either as they are usually all made to fit the general palate.
I think probably if you are looking for a cheap alternative, try and get some special roasted beans for the cold brewed coffee and then use what ever brand of coffee takes your fancy for the grounds.
Also, have heard that people can get green pepper flavours out of coffee grounds.
Hope this helps!