I think Jeff covered most of it, but you might want to also try, (
www.surfsource.net) they carry every thing you may want, even cut block (24" wide) 2pcf EPS blanks. Retail prices are pretty good, some materials are less from fiberglasssupply. Foamez is a good supplier for home builders (or if you live in the area), but wow are they expensive, IMO...... If you have room to store billets, cutting your own blanks is probably the most cost effective for raw materials... it is less cost effective, when you consider your time and effort. Billets vary considerably in quality (from a surfboard building perspective), they tend to vary in density, bead size, and particulary bonding or fusion of the individual beads based on the steam expanding process. We were cutting billets in the 90's largely to build better boards and out of frustration with clark.... however, now there are a few companies that offer pressure fused blanks that are more similar to typical close to shape blanks and modern CNC machines make tweaking and replicating designs effortless..... Also a good hot wire cutter or bow will cost you, unless you tend engineer up your own, then you can save probably $100.
I can cut you a deal on fins/boxes/plugs and goretex vents, etc if you need.
Sealers are great.... but have drawbacks. An epoxy seal is more durable, but if not careful you may end up with high spots that will otherwise marr a clean/clear finish board. Keeping in mind than foam and resin sand at much different rates..... Light weight spakle is awesome (at first), blends great, smoothes everything out, but there is a fair amount of shrinkage while drying (may require more than one coat). Most manufacturers only use spackle to fill bad gashes or if an EPS board needs a spot on spray paint job.... (Polystyrene does not paint clean and nice like Polyurethane, when it comes to water based acrylics, spackle helps but may again leave a less than desirable finish). Light weight spackle also tends to yellow over time, this is compounded (visually) when cheaper resins are used. Some go ahead paint the whole blank white to blend the colors after spackle (white paint is not the same as white foam for a finished board).
If you plan to have a mostly white board (raw foam), then learning to shape and finish it without needing a filler/sealer is ideal.. but not always easy with EPS... it is all about pull versus push and whether to sand versus tear/rip.... you will want to take it down to a 220 grit if painting or to really smooth it out...
Next we can discuss the wild array of fabrics, and laminating materials...... oh the fun....