I'd say at least 3m, if not 4m - though I have seen a lot of variation with sling 'tails' in terms of length.
Here is the Slingbabies page on ring slings which has a good overview:
http://www.slingbabies.co.nz/Site/Ring_Slings.ashx
I'm not sure whether the rings are 'easy' to find, I did a quick google and there weren't heaps available directly in NZ, and I'd personally rather source metal rings over nylon ones. You could try and overseas vendor though.