They are just so bloody expensive... is it just a long time accumulation or is there a place (online or otherwise) that needs some wider recognition?

Shapers on the Gold Coast.
Heaps of options at a good price.

Shapers - its like walking into a lolly shop.

i have bought stuff over the net from them as well

Chuck all the big surfboard fin brands make blank fins for every size thruster fin model that they stock. So either befriend a shaper or head to a local surf shop owned by one. In Sydney Simon Anderson has one, manly, One at mona vale run by old division shapers, Brookvale has bennet and also another two shapers close to bennets in broojvale near the pacific highway. Lots of options in Newcastle and wollongong. So for your surf set up and side fins you should pay around $20 a fin, those larger back fins are not cheap so around 80 -100 and these are usually scarce as the industry does not manafacture them by the million.
Heads up for the surf shop in Bondi who helped me out when I broke both my plastic starboard side fins,

www.hugedomains.com/domain_profile.cfm?d=indofins&e=com has good prices for old style fcs fins in fiberglass.

True Ames from the US.
Rainbow Fin Company also.
Alkali from Balina my favourite.
Captain Fin.
But they all cost.....
Although you can get True Ames shipped from the US cheaper than you can buy FCS here, and they are a really nice fin.