I want to buy a chartplotter - but the electronic charts are hugely
expensive, and also difficult to buy as not many chandlers etc stock them.

What is the best solution? Is it possible to buy a CD of the world and then
download the charts one needs into the chartplotter?

Surely the first manufacturer to offer a solution for easily and relatively
cheaply loading charts into the chartplotter would corner the market - so
what's stopping them? The charts themselves can't be too expensive as iirc
the US Hydrographic office publishes free charts for the whole world.