There are many ways to get advantage/disadvantage on attacks: the Dodge and Help actions are specifically designed for this in combat, and 10 of the 14 conditions in Appendix A have at least one effect causing (dis)advantage on attacks. Also note they use the universal term "attack roll" with no distinction between melee/ranged or weapon/spell.
By comparison, effects on saving throws seem to be quite rare and limited in scope. In terms of common actions & conditions:
- The Dodge action gives advantage on Dex saves;
- The restrained condition gives disadvantage on Dex saves;
- 4 conditions (paralyzed, petrified, stunned, unconscious) cause Str/Dex saves to fail automatically, in addition to other debilitating effects.
This still leaves 4 of the 6 ability score saves unaccounted for.
What options are readily available for a PC to penalize a specific saving throw? For example, in order to make a spell that requires that save more likely to take effect.
For the purpose of defining "readily available", I'm looking at these criteria:
- Either applies universally to all saves (such as "the next spell you cast that requires a save") or lets the PC choose from more than 1 ability score save.
- Is available to PCs below level 5 (doesn't require dedication to a particular class beyond the first advancement tier).
- Appears in any officially-published book (not Unearthed Arcana or third party).