Valves
Boston valves: I searched around to see if I could find a cheaper Boston Valve, no luck in New Zealand, all I managed to find were replacement inserts, not the flanged valve itself. I ended up buying from Aliexpress for $6 USD a piece with free shipping. They seem to be good quality, I ordered three and they all came as yellow ones.
Top Off/seat Valves: Once again my search had limited success, I found spare valves for NRS kayak buoyancy bags for $1.49 NZD a piece which fit onto I think 8mm clear PVC tubing but finding a flanged fitting to attach it to the raft has proved difficult. I then realised that inflatable lifejackets have a very similar valve that can be cut out and repurposed. Life jacket service agents might give you these for free (I got four from RFD/Survivortec), if they come with the lifejackets then there may be other useful fittings (buckles, D-rings and clips) as well.
Inflatable mattresses (e.g. that holey Thermarest in the back of the cupboard) also have valves that may be useful.