When you true polymorph into a monster you gain its usual equipment
Note that the true polymorph spell's description states:
The target's game statistics, including mental ability scores, are replaced by the statistics of the new form...
The question then becomes: "What count as creature's statistics?". Luckily, there is already an answer to that question here which has 20 upvotes, no downvotes, and is accepted, leading me to believe it is generally agreed upon. The answer uses the following quote from the Monster Manual (page 6):
A monster's statistics, sometimes referred to as its stat block, provide the essential information that you need to run the monster...
This allows one to define the following: If it is on its statblock, it is part of the monster's statistics.
The Monster Manual goes on with short sections on each part of a creature's statistics (pages 6-11):
- Armor Class
- Hit Points
- Ability Scores
- Saving Throws
- Vulnerabilities, Resistances, and Immunities
- Special Traits
- Limited Usage
A monster's equipment is included on its statblock and thus is included in its statistics. This is further supported by the simulacrum spell which states:
It appears to be the same as the original, but it has half the creature's hit point maximum and is formed without any equipment. Otherwise, the illusion uses all the statistics of the creature it duplicates, except that it is a construct.
There would be no reason to say that equipment are not created if equipment were not part of a creature's statistics. Thus, equipment are part of a creature's statistics.
true polymorphing into a monster will give you their usual equipment