Specifically, any that you can find fairly early on?

I have a great solo character for BGI (A Berserker 7 > Mage who is a Grandmaster in Scimitar), who I'm about ready to import into BGII. But I don't think there are any very good Scimitars to be found in BGII aside from Belm +2. I know many, many monsters need a +3 weapon or better to hit.

If there are any, where can they be found?

Have a look here.

WANINJA.ITM, The Scarlet Ninja-to; +3 and adds an extra attack.

This, of course, is sold by Joluv, the bonus merchant in the Copper Coronet.

The Scarlet ninja-to only usable by monks or high level rouges with use any item.
The only +3 scimitar in the SoA is the Kachiko's Wakazashi.

There is another plain +3 scimitar in Underdark, at the Jaellat house, and there would be another in my mod, which I scribe, and hopefully before Summer will the final release

We are, of course, forgetting Usuno's Blade +4 (from the first level of Watcher's Keep).

@WizWom
You are the man!

The problem there is that it's a tough fight to beat those statues, and I don't think the one holding Usuno's Blade spawns until you've hit a certain level.

Yes the amount of statues is contact with your experience so if you go to watcher keep on low level you only get 1-2 statue and you can miss the usono blade, if you go to chapter 6 you able to gain it. I will test it once.

Thank you Klorox! If you didn't write this i go low level to watchers keep (there are good stuffs in it). In the chapter 6 one more +4 weapon will be very good because i play with IA and there are monsters who immune to +3 weapons.

There are monsters in Watcher's Keep level 3 that drop Scimitars +3 and Chain +2. The scimitar has the same description 'Water's Edge' as the Scimitar +3 you find in the Underdark. I guess it wasn't unique.

I can't remember the name of the demons that carry them. Their sprite looks humanoid but they aren't succubi, erinyes or tieflings. I was resting and I was attacked by 5 or 6 of them.

That's Alu-Fiend.
Also Yaga-Shura's Lieutenant Fighter and Saemon Havarian in Amkethran have Scimitar +3: 'Water's Edge'.

Good +3 bastard swords are rare, too, as I recall.

Until you get to the Underdark where you can aquire 2 of them