I am in need of classical eigenforms for $\Gamma_0(p)$.
In particular I need these to be newforms for each of the even weights 8 to 26 and would settle just for cases $p=2,3$ at this moment in time. I need them in a format from which I am able to find critical L-values.
Does anyone know where to find these and/or whether MAGMA/Maple/PARI is able to compute them? (The online documentation for MAGMA doesn't seem to mention commands for finding eigen-bases)
I think the best place to look might be the new "L-functions and modular forms database" website (still in beta at the moment). This has very detailed tables of modular forms, and, better still for your purposes, about their L-functions (critical values, first few zeros on the critical line, etc.); e.g. check out http://www.lmfdb.org/L/ModularForm/GL2/Q/holomorphic/7/4/0/a/ for lots of info about the L-function of the weight 4 level 7 eigenform.