Can anyone tell me wherethe heavy crossbow of accuracy is? I can't find it anywhere!!!!

Here's what to do: kill Aldeth Sashenstar either in Cloakwood (part 1) or in BG city (in the Merchant League). He's the guy who had troubles with the Druids, remember? Now, after you killed him, travel to the BG center map. Fairly to the south of that map you will meet Aldeth's bro, Daborn. He wants revenge - which he obviously won't get. Pick the Heavy Crossbow of Accuracy from his corps.

Petrarcus ap Bhaal.

BTW: it's a great item, superb THAC0-bonus (+5)!