For making a archer, I personally think the most important stats are in order from most important to least...
Dex For both the bonuses to hit with missile weapons and the AC bonus.
Con Obviously for more hit points.
Wis Having a high wisdom gives you a better magical defense adjustment. A 18 in wisdom gives a +4 bonus to your rolls to save against magic... Don't be Minsc, have high wis
Str I place str lower on the list than maybe some people would, but when it comes down to it, it won't be too frequently that you will have to rely on melee weapons... Especially if you have a party to keep the opponents back from you. Also, there are several strenght enhancing belts, and spells avaiable in the game.
Int & Cha Neither of these really play a roll for archers, but they are nice to not have penalties in, so if possible keep them at 8 and above...
Spoilers
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You can find a nice longbow in the central area of the upper level of Nalia's keep. The treasures are guarded by several golems, and they are rather tough early on, but you can grab the loot without having to fight the golems if you have a cleric cast sanctuary. Then he/she can just waltz in and take the treasures.
There is a good Shortbow available in the Copper Cornet, if you go into the back and free the slaves, the key to the slave's cell is held by the beastmaster who also has the tugian shortbow, which lets you make an extra shot per round... Yum.
Most of the good crossbows seem to be on merchants... But you can at least get the heavy crossbow of accuracy off of one of Gaal's guards when you complete the cult of the eyeless quest... (go back and they jump you for destroying their god). If you have ToB, you can save up and go over to watcher's keep... The merchant on the roof will sell you a very nice no-ammo-needed crossbow.
Oh, last thing I can think of to say is, Don't miss out on crossbows... There is some quite nice ammo you can find for them, especially the Kuo Tao bolts you find in the underdark... Extra yummy.