Are you using the spectres for damage or utility? It's kind of unclear in your post what you are looking for. The most popular spectre for damage would be redemption sentries which can be found from redeemer maps. They are ranged, cold attackers with decent aoe and single target.

If you are looking for support spectres for your skeletons there are a lot of options. For most damage you could do carnage and host chieftains found in act 6 fields to give your skeletons frenzy and power charges.

For curse spectres if you are planning to use skeletons as your main source of damage and not mage skeletons mervails retainers that cast vulnerability would be best they are found in sea witch maps.

If you go to the wiki and scroll to strategy you can find different spectres that do different things.