Spells
Most spells that are good for Wizards or Sorcerers are good for you too. No spells really jump out at me for being particularly strong for a Sha'ir more than they are for a different arcane caster.
These spells are available either early or not at all to Wiz/Sorcs but are on your list of domain spells. Research provided by Daniel678 from the BG site.
1 Magic Stone
2 (Wiz 3) Wind Wall, Soften Earth and Stone, Produce Flame, Calm Emotions, Aid, Heat Metal
3 (Wiz 4) Stone Shape, Searing Light
4 Air Walk, Chaos Hammer, Spike Stones, Divination, Order's Wrath, Freedom of Movement, (Wiz 6) Control Water
5 Control Winds, Dispel Law/Chaos, (Wiz 6) True Seeing, Flame Strike
6 Animate Objects, Fire Seeds, Find the Path
7 Word of Chaos, Earthquake, Fire Storm, Dictum, Sunbeam
8 Whirlwind, Cloak of Chaos/Law
9 Elemental Swarm, Miracle
Of these Calm Emotions can occasionally kill Barbarians, Freedom of Movement is very strong, Air Walk and Control Winds are good as well. Miracle makes Wish look like garbage.
The Elephant in the Room
Diplomacy, the skill check you have to actually make in order to cast ANYTHING. How do you do it?
Like so:10 (take 10)
4 (ranks in Diplo)
3 (charisma mod. Might be 4)
2 (It's a spell known)
1 (You are a first lvl sha'ir)
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20
One would still need a +2 boost at first level in order to always make the diplomacy check. (And you have to basically check for your first level spell every hour, so rolling for diplomacy is a bit risky since the -2 failure penalty is cumulative for the rest of the day.)
If you can get a +4 boost somehow, then you can pick any 1st lvl spell (that you've seen and identified with Spellcraft), not just the ones 'known'.
~ Epimetheus
My recommendation for that would be a Masterwork Item that gives +2, and costs 50gp. If your Gen then aids you (by taking 10) you have successfully gotten your +4 and can always pass the diplomacy check. The DC actually gets easier over time so you should be all set.