TA Performance is another supplier. Even Rockauto offers many stock parts.... it's a great site to get part numbers and initial pricing from.
I've bought the components separately from various sources.
Sealed Power makes main bearing sets, pn 4265M. It takes some searching to find them for under $250.
Rod bearings are the same as the 400-430-455. Clevite for those.
Dura Bond cam bearings.
Timing chain and gears, Cloyes and Melling offer stock units. Upgrades avail from TA and the Martins.
Gasket sets.... I've used Fel Pro but have heard 'Best' has a nicer set.
Pistons are tough since nearly all have a lower compression height that moves the piston down in the bore which reduces compression. Best solution is to get a custom made forged set from Tom. They're pricey but are the best offering out there.
Camshafts are offered by several suppliers, TA Performance, Schneider, Comp cams (get a custom grind, NOT the Thumper series!), as well as some lesser known companies such as Camcraft which has been highly reccomended. As to which is 'best' is tough to say since there has been little actual dyno testing done to compare. The TA 25 is a nice upgrade from stock for a street car. The rear axle gears and engine compression should be matched to the cam for best performance.
Valve-piston clearance is tight in Nailheads which can limit cam choice.... unless you notch the pistons for clearance... another reason to spend extra for forged pistons that will be made to fit your build.
I haven't talked to Carmen in decades but he certainly has the knowledge to get you the best parts. Wish he had a net presence! Tom certainly has the knowledge as well! The Martins offer some nice products but IMO Carmen or Tom have a higher degree of technical expertise.