At often times, a good held item for Gen 3 playthroughs would be something that raises the Pokemon's STAB power. Though knowing the Sharp Beak and Poison Barb are hard to obtain, here's your other options.
Kings Rock - Despite being obtained late in the game (at Mossdeep, right before fighting Tate & Liza), the fact that Crobat can outspeed most Pokemon, the flinch chance, if activated, can prevent it from taking any damage!
Sitrus Berry - While an odd choice, it can be used until you get the Kings Rock, being obtained somewhat early (in Routes 118, 119, and 123), the small recovery can help Crobat last longer, because of it's mediocre defensive stats. Shell Bell may be a better option if you DO have it.
Those are really the only suggestions I have, if you've got Sharp Beak and Poison Barb, you may also use those.
Source: Experience, and Bulbapedia's Item Locations.
I hope I helped!