Constant, Scaling Bonus

The only other item that boosts spell save DCs by a constant amount is the Robe of the Archmagi (DMG 194), which gives a +2 to spell DCs and attack bonuses.

Effectively increase DCs by using items with fixed DCs

Depending on what your save DC is, you could use various magic items to increase your save DC on certain spells.

For example, if you feel like taking a risk, you can try to cast a spell from a spell scroll that's a higher level than you can ordinarily cast (DMG 200):

If the spell is on your class's spell list but of a higher level than you can normally cast, you must make an ability check using your spellcasting ability to determ· whether you cast it successfully. The DC equals 10 + spell's level.

The table in the description shows that spells cast at higher levels have fixed DCs, so if you cast a 7th level spell from a scroll, its save DC is always 18.

Likewise, many (but not all) magic items usually have fixed DCs for the spells that you can cast using them. For example, the Helm of Brilliance (DMG 173) has a DC of 18 for all of its spells regardless of the caster's stats:

You can use an action to cast one of the following spells (save DC 18), using one of the helm's gems of the specified type as a component: daylight (opal), fireball (fire opal), prismatic spray (diamond), or wall of fire (ruby).

In this way, if you can get your hands on these magic items, you can effectively cast specific spells with a higher spell save DC. While a single-classed spellcaster that's maxed out their spellcasting ability probably won'tThis strategy works best if you can get much of a benefit from thatthese items at lower levels, a character with worse ability scoresif you're multiclassed, or multi-ability dependency might benefit moreotherwise have lower stats.