The Ship Model Forum

The best Ive found is Blick art materials.
I bought lots of foam blocks from them at a decent price.

Don't put fibreglass immediately on top of the foam though. This technique is often used in scratch building submarines. Put different sections (cut to the largest diameter of the section) on a central stick. Then sand or cut to shape.
Before fibreglassing they do put something aluminum foil or some sort of tape over the shape, otherwise the resin eats or melts the foam while settling. Not entirely sure what they do/did, but looking at their technique will bring you more advice I'm sure.
For cutting the foam they often use a melting wire. Melts its way through rather than actual cutting, gives a smooth finish.

Try your next or second-to-next DIY store (not all of them carry the needed shape or grain). Foam board comes in various thicknesses, at least here where I live...

Happy scratchbuilding ~ Olaf!

I was wondering if you can used foam board blocks for ribs and glue them together then cover if with fibreglass to the shape of the hull and where can I get the foam board?

Graham