[mothered]

How would I go on about finding the exact weight for the shirt and belt if it’s not listed on the website?

Amazon lists Item weights of almost every product, so see If they're listed there.

Alternatively, simply email/call the company and request a breakdown of each one. If they ask why, tell them that you're going on a flight and will be taking the goods with you, hence need to work out the baggage weight limit.