First of all, the
Soul Preserver is one mighty fine trinket, regardless whether you're disc or holy. It'll be a little bit nerfed with the mana regeneration changes (mana gained outside the 5SR will be lowered), but it'll still be one you can easily keep 'til you start Naxx 25. As another plus, it drops from normal CoT: Stratholme. A random drop in heroic HoL is the
Winged Talisman which will retain it's usefulness past the next patch. Also you might want to start running heroic HoS; both the
Spark of Life and the
Forge Ember drop from the last boss Sjonnir there. While the Forge Ember is undoubtfully better for disc priests, it's still a fairly nice trinket to have when starting Naxx. Last but not least there's the
Talisman of Troll Divinity that drops from Tharon'ja in heroic Drak'Tharon Keep.
If you want my opinion, I'd focus on getting the Soul Preserver first and the Spark of Life second. This is because even with the upcoming regeneration changes those two will grant you the highest and imo most useful proc/triggered effects. The thing is that the Soul Preserver is an effective ~128MP5 over the course of an average fight, while the Spark of Life sits on ~52 and has the fairly useless haste stat to go with it. That said, the Spark is
still the most powerful out of all other named trinkets; The Winged Talisman is on 29MP5 and has a fairly useless use effect, and even though spirit is of more use to holy priests than it is to disc priests, the Talisman of Troll Divinity is sitting around 20MP5 and with that has the lowest MP5 out of all regenerative trinkets in this post. The Forge Ember has an unreliable spellpower proc and it's main use is the crit rating on it - it's more of an "if all else fails" trinket.
If you want the proper math behind those numbers, just look up the items on Wowhead and check the comments.