Basically, core spells make up the largest percentage of spells. Very few spells from outside of core are ever allowed.
I consider really carefully before adding new spells. One of 1e's/2e's problems was it eventually had too many spells; if you added all the spells from supplements, 3e had the same problem. If the spell serves the same purpose as an existing spell, but has a slightly different flavor, I generally don't add it. Likewise, if a spell is a reflavored version of another spell from a different school, I never add it. One of the things that broke speciality wizards in 2e was eventually you weren't giving up anything, because every school could do basically everything. It's really easy to imbalance the game with spells.
Most of the relatively spells I add to core I tend to make up myself, mainly to fill niches that I don't think are filled by the existing spell list.
Typical niches:
1) Divination based self-buffs. (Because low level diviners don't have enough offensive options)
2) Cantrips (Because I love low level spells)
3) Low-level shape changing spells. (Because low level shape changers don't have enough non-broken options.)
4) Spells translated from their psionic equivalents where an equivalent doesn't exist in arcane magic. (Because I don't use psionics as a separate system.)
5) Low level telekinetic effects.
6) Low level long term buffs for objects. (Helps create continuity with the real world if low level magical attacks can be more easily defended against.)
7) Low level defensive magic keyed to an area. (As #6)
Conversely, many spells from core are weakened in some fashion, particularly existing shape change (alter self, polymorph self, polymorph other), absolute defenses (death ward, extradimensional space effects (rope trip, mord's mansion), force effects (wall of force, force cage), information finding divination spells (most detects, commune, find the path), and battlefield control spells (entangle, web).
The main 3rd party spells I use are cantrips. Pretty much any balanced and useful cantrip will be added to the spell list.