I heard that men whose daughters are dating have a mitzvah to supply them with things that will help them, i.e. dresses, jewelry, etc. Is there a source for this?
The Talmud in Kesubos (52b) states:
מצוה דנלבשה וניכסה וניתיב לה מידי כי היכי דקפצי עלה ואתו נסבי לה
It is a Mitzva to clothe her, cover her, and give her stuff, so that men will jump at the chance to marry her.
The three things mentioned presumably refers to clothing, jewelry, and money.
Although the Gemara there says that it is a Biblical mitzva, see Ritva that the intention is rather a mitzva found in the prophets' writings and thus is only mDrabanan.
The Gemara in Kiddushin says it's a mitzva for a father to attempt to marry off his children. The Gemara then says, "for boys, that's easy. But how do I get someone to marry my daughter?" Answer: it's a mitzva to offer a nice dowry, to attract a boy to marry her.
This is codified in Shulchan Aruch and commentaries there; if I recall correctly, a young woman can sue her parents if they're making zero effort whatsoever to help her get married, but as long as they're making some effort, that's all that's enforceable. Similarly, if she meets a nice young man on her own and they like each other enough to gasp! get married without her bringing in any money, that's totally fine too. However, anything her parents do that would genuinely help her chances at getting married would be considered a mitzva.
In a world of bottom-line focused marriages, offering a nice dowry seemed to do it. If you argued today that what does it better is nice clothing, it wouldn't be unreasonable. The bottom line is, for a son or daughter, it's a mitzva to try and help them get married -- assuming they're trying themselves, and assuming it will lead to a happy marriage.