The original GM performance cams are still available from GM Parts Direct
These cams are manufactured for GM by Crane Cams and are also available directly from them. Call them or go to their web site to find their part number(s).
Summit lists and sells the Crane replacements.
The original RPO-L78 (396/375) cam was part #3863148 - the new GM part number is #12364057
Cam spec.s are;
Duration @ 0.50" = 242/242
Lift = .520/.520
Lobe Center = 114
You can of course go to a 'Bigger' cam, but be sure you discuss the application with the cam manufacturer as it may require some other changes.
Trying to go to the RPO-L88 type cam for instance usually requires a compression increase and other changes to take advantage of the cams grind.
There are also some really good Hydraulic cams that deliver nearly the same results with less upkeep than the original Solids.
The GM/Crane #12353922 (@0.50"=218/228-lift=.500/.500-114) works well with higher compressions (9.5~10.5:1).
Discuss your engine set-up with the cam advisors to get a recommendation closer to your exact needs.
BTW - these cams require the GM performance valve springs.
Make sure your springs weren't changed to accomidate the Hyd. and are no longer compatible with a new Solid cam, bad things could happen ...
Hope some of this helps