These are elemental spirits, and can increase the power of characters that hey combine with. Unfortunately, you won't find three of them until you're near the end of the game, which means you won't be able to use most of the special forms until you're at the end. So here they are...
Sana the Fire Shaman
You'll find her in Capitan, after you've received the spell from Ray. Just talk to her. When she combines with someone, she'll raise his/her attack power.
Seso the Water Shaman
You'll find her in Nimufu's Tower, once you've beaten Nimufu. Back-track from Nimufu's room to the chamber with those five moving platforms on tracks. One of the two platforms on the right will take you to the ledge on the left. Continue from there until you find her. When combined with someone, she'll raise his/her Wisdom.
Spoo the Wind Shaman
She's in Sky Tower, and you have to find her before you'll be able to enter the valley leading to Farm Town. To reach her: From the entrance, head west, and take the north exit. Go up the steps, and then take the south exit. Go along the passage, go through the exit at the end, and take the stairs. Go through the exit, go through the passage and take the exit. You should now be in a room with four strong currents surrounding some stairs. Make your way to the steps and go up. Keep going up until you reach her. She'll raises the speed of whomever she combines with.
Solo the Earth Shaman
Located in Farm Town. When Daisy tells you to go pray for a good harvest, head to the shrine of Namanda and donate 2,000 coins (at least). You'll find her in Diasy's field after either Daisy's death (maybe), or after the fall of Evrai (definately). Solo raises the defense of whichever character she combines with. Note: According to popular belief, you must also NOT use any attack magic during the the clearing of Daisy's field, or in the fight with the Evrai warrior in the field later, in order to get her. Ben Siron says this is BS. I'll try using attack magic in the field my next time through the game, and see what's true.
Seny the Holy Shaman
After the fall of Evrai, return to Bando. She's waiting for you in the first cell in the area with the moving corridor. Seny can't raise any of your abilities herself; a special form must be the result in order for attributes to go up. This is appearently a bug, and Ben Siron's manual lists it as such.
Shin the Devil Shaman
Enter the left gate to Infinity (it had a claw over it) in order to find her. She'll raise the AP of whomever she combines with.
Special shaman combos are covered under each character's profile.