Hi Erik,
I purchased my pultruded carbon rod from Jim Marske in the US - good price and excellent service. You should also buy a copy of his Composite Design Manual if you are going to build a wing using carbon/kevlar/glass fiber and epoxy - it's an essential primer if you're new to this sort of thing. He includes a section on different methods of spar design and fabrication. I used Marske's formulae and method when I created my wing load/stress analysis spreadsheet. You will also discover how to design and fabricate tapered carbon load transfer bars to transfer the spar cap loads from the pultruded carbon rods to the metal wing connection fittings and bolts.
Study both the Design Manual and my spreadsheet to get a better understanding of the loads and stresses in the wing spar and how to calculate the amount of material (rods and twill cloth) needs to be used to carry those loads.
I designed my spar to take 10.5g simply because the materials could do this! All the spars in my wings/tail/rudder/H-stab/elevator are made from two layers of 195gsm twill carbon cloth with the pultruded carbon rods fully encapsulated between the two layers of cloth. Technically only one layer of twill cloth is required to carry the shear loads, but this makes for a very fragile spar web which can be easily damaged during the rest of the wing construction proess. Even with two layers of twill cloth, my spars are lighter than the plywood originals.
Be careful when choosing your carbon cloth - you want a cloth with a 'medium to high' modulous (stiffness). I made a number of test pieces with different grades of CF cloth, all with roughly the same gsm weight (grams per sq-meter) and was amazed at the difference in stiffness. Low modulous cloths (mostly uni or bi-axial) resulted in very bendy (not stiff) shear web and leading edge test sections. You could still use these cloths but you would need to use a 2mm sheet PU foam core between the two layers of cloth to add stiffness. It's simpler (and lighter) to use a higher modulous cloth.
Stiffness in your leading edge cloth is especially important - you don't want it flexing or deforming when you're flying at high speed!
Feel free to pick my brains - I'll give you all the help I can.
All the best,
Phil.