I recently bought some Noodler's Black and Walnut inks from noodlers.co.uk. I've had good results with both (though my pens do get an airlock from time to time, easy to clear, but annoying to have to). Noodlers Black is 'bulletproof', i.e. once on the paper then it stays on. Yet it is fountain pen safe. There is even a challenge on the US site to the first person who can reliably remove it. There is also an eternal blue due to be released.
There are other 'Bulletproof' inks, such as Eternal Brown and Legal Lapis, both of these are only available through one supplier.
That said, Walnut was pretty good when I tried to remove it with an ink eraser (which simply makes Quink and the like vanish).
I've now placed an order for Legal Lapis, this is not available in the UK and so it's not cheap, but if it is as nice as the others, I am looking forward to getting it.
These inks (and the accompanying fountain pens) are not cheap initially, but over time they fair well compared to the cost of disposables (assuming that one doesn't acquire disposables through the office...!)
There is also something almost sensual about using a fountain pen which is not replicated with your throwaway biro.