Clarke Foam is what I used in my projects. You can buy blanks from Seabase in Newquay (minimum quantity is 1

), and everything else that you might need for your DIY board building excercise.
http://www.seabase.ltd.uk/
I had great experience in dealing with them, and they even took me to their factory to see the processes involved in making boards, and get all the advice I needed.
Infact, they sell three complete kits (from shortboard to longboard), and they include:
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Seabase Surfboard Kit - the original and still the best!
If you are starting out, why not try one of our famous Seabase Surfboard Kits? They contain all the materials needed to make a surfboard. We also include full instructions on how to make your surfboard.
Each Seabase Surfboard Kit includes the following:
1st Quality Clark Foam Blank
Hexel Fabrics Fibreglass in 4oz or 6oz - you choose
ISO 7x Laminating Resin (4kg or 5kg depending on the blank)
Taupo Gloss Resin (1.2kg) - optional
100 ml Catalyst / Hardener
125 ml Wax in Styrene (for sanding / hot coats)
1 Deck / Leash Plug
Choice of Fin Systems & Fins (Euro Fins / FCS, Futures Fins, Speeedfins, Glass On Fins, and more)
Clark Foam decals
Seabase - 'Making a Surfboard' instructional leaflet
Next day delivery*
Those are all the materials you'll need to make a surfboard.
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Hehe, great stuff. Now, you just need to find a place where to attempt this build. I did mine in my spare bedroom, but I must admit - that was daft. Still, it produced some great results

.
But I also uncovered the ugly side of board buisiness. Surfers Paradise shop would tell me that they owned their own board factory. I then go to Seabase later on the same day, just to find out that Seabase makes boards for many shops. I dont mind this, but why lie. Also, lots of shops will tell you that they imported board from, say USA, when it has been made localy by somebody. Grrrrrrrr

.
In other words, you want to buy Rusty surboard, thinking it is made by the great man himself over in the States. Or Hawaiian Island Creations board. Think again....