Where are Golden saints? I need them for constant effect enchantments, but they are very hard to find? Also, are there any others souls (not including the gods) that can be used for constant effect enchantments?

They're mostly level dependent (after level 17 IIRC) so the best bet is to summon them if you're lower level. There are a few fixed GS; one in Bal Fell and Staada (named GS in Azura's quest). You can also use Ascended Sleepers but they're even harder to find than GSs.

If you are level 17+ go the Grazelands and go on Daedra Hunts. The last two times my current character went on a Daedra hunt she bagged four Golden Saints on each hunt. That was eight Grand Soul Gems with Golden Saint souls and I could have had sixteen if I did a certain glitch right and had enough Grand Soul Gems. I like going on Daedra Hunts because there are other Daedra that spawn in the Grazelands that I can use in alchemy such as void salts from Winged Twilights for Restore Magicka potions. You will have to walk around until you know where the Daedra spawn points are. After you know where they are it does not take long to bag your limit of Golden Saints.

Ascended Sleepers are your other choice. You can find them in Sixth House bases and they respawn there.

There are some other unique souls that can be used for CE enchantments but I usually just keep them in my soul gem collection.

Golden Saints

Leveled creatures, which means you will find them randomly when you're around their level. Golden Saints are level 20.
There are certain places where you'll find them regardless of your level:
-Odrosal, Dwemer Training Academy
-Ibar-dad
-Bal Ur, Underground
-Ald Daedroth, Inner Shrine
-Ald Daedroth, Antechamber
-Azura's Star quest (unique GS)
Levelled creatures in the Grazelands and Molag Amur
Randomly around Daedric ruins starting when you reach level 18.

Summon Golden Saint spell: Felen Maryon, Upper Tower; Therana's Chamber, Tel Branora.
Summon GS scrolls: Folms Mirel, Mage's Guild in Caldera (you need to be a conjurer of the mages guild before Folms Mirel will deal with you.)
Hlendrisa Seleth, Tel Uvirith (after strongholds built)

http://mondgesaenge.de/MGES/morromap/index.htm

You will also find them in the Molag Amur Region, in the direction to Mount Kand and to Mount Assarnibibi, starting from Molag Mar.
There are other souls to capture yes, beside the demi-gods of the temple and the Aureals (golden saints)
Dahrk Mezalf, an unique dwarven spectre. You get a quest from Baladas Demnevanni to fetch a dwarven ring from the corpse of Dahrk Mezalf if you belong to House Telvanni and have reached the rank of mouth.
Dahrk Mezalf can be found at Bthungthumz. Another option is Ascended Sleepers. Ascended Sleepers can be found at Sixth house bases. There are also some dagoth's which are an ascended sleeper, like Dagoth Tanis.
And that's pretty much the options you have.