[RPG] Are there higher CRs possible than the Tarrasque’s CR 30

Using an online Challenge calculator, I am not seeing the issue that you are describing.

Monsters exist up to CR 24, and then the Tarrasque by itself is 30.

For 4 Level 20 PCs, a single enemy of CR 23 is at the high end of a Hard encounter, just barely below the threshold for a Deadly encounter. A CR 24 is thus a Deadly encounter, but 5 Level 20 PCs push it back down to a Hard. (The Tarrasque requires 7 Level 20 PCs to push it under the Deadly level, but lets ignore that one! Using that, you are definitely threatening a TPK!)

I believe you are calculating challenge improperly. I will explain what I think you are seeing, and you can obviously correct me if I am off base.

For the CR calculations(using the table on page 82 of the DMG or page 56 of the Basic DM Rules), a level 20 PC has a Deadly threshold of 12700 XP. A single CR 15 monster is worth 13000 XP. Since that is higher, you see it as being a Deadly encounter. However, for encounter calculations, you must add in the XP threshold for each party member to arrive at your overall thresholds (which is explained in step 2 of the Evaluating Encounter Difficulty section on the same page as the XP Chart). For a party of 4 Level 20 PCs, the Deadly level is set at 50800 XP. A single CR 15 monster against a party of 4 Level 20 PCs will be an Easy encounter.

In 5E, there are only a handful of sources for magic items that are official - primarily the Dungeon Master's Guide, and the appendices of published adventures.

After looking through all of these, the answer to your question is: No. The Blinkback Belt does not have an exact equivalent in D&D 5e.

There are a few magic items that have a similar effect though (in that they can be thrown multiple times). However, all of these return the item to your hand, not your belt - a small but perhaps noteworthy difference.

From the DMG:

• The Dwarven Thrower (Very Rare Warhammer, only usable by a dwarf) can be thrown, and immediately flies back to your hand afterwards.
• Whelm is a legendary sentient Warhammer (only usable by a dwarf) that has the same feature

From the Princes of the Apocalypse adventure:

• A Storm Boomerang returns to your hand if you miss (but not if you hit)

Finally, the closest to what you're looking for is probably in the adventure Waterdeep: Dragon Heist:

• The Bracer of Flying Daggers functions similarly to the Blinkback Belt (albeit more limited in what weapons can be equipped to it, but allowing an additional attack):

  This armband appears to have thin daggers strapped to it. As an action, you can pull up to two magic daggers from the bracer and immediately hurl them, making a ranged attack with each dagger. A dagger vanishes if you don’t hurl it right away, and the daggers disappear right after they hit or miss. The bracer never runs out of daggers.