Hmmm, can't say I've ever seen the seeds for sale, and my two plants are not seeding (yet). You probably all ready know that M. carminea grows as a climber if propagated from seed, but cuttings taken from an adult have shrub-like growth. I suspect most gardeners would prefer to have the shrub-like plant rather than a huge climber due to space restraints, hence making cutting-grown plants far more abundant than what the seeds are.

All this searching has got me curious, now I want some seeds too! All my native books say that seeds are produced freely and are easily grown.

Anyway, here's a few websites you might like to try, but I'll still keep an eye open for seeds for you:

NZ seeds have seeds of M. robusta as opposed to M. carminea, but if you email them, they may be able to help you out:
http://www.nzseeds.co.nz/

This is a webpage of mail order seed nurseries that may be able to help you as well:
http://netlist.co.nz/gardens/mailordernurseries/directory_list.cfm?CatID=41&Category=Seeds

Same deal as above, but this links to a list of native plant nurseries:
http://netlist.co.nz/gardens/mailordernurseries/directory_list.cfm?CatID=33&Category=NZ Native plants

Good luck!
Sarah