In AFK Arena you can acquire 3 types of Soulstones: purple Elite soulstones, elite and rare Hero soulstones and blue Rare soulstones. You need 60 of them to summon Elite hero from Elite ones or Rare hero from Rare stones. Hero soulstones are currenly available only in Store and summon certain hero (e.g. 60 Shemira soulstones summon an elite Shemira).
While summoning elite hero you should know, it's not necessary has to be ascended tier hero (e.g. you can get Elite Vedan from 60 Elite soulstones).
A steady source of Rare soulstones can be acquired from the bounties. Depending on bounty rarity players can get different amounts of Rare soulstones.
Great source of both Elite and Rare soulstone is the Barracks store. You can buy there 60 Elite soulstones for 18 000 Hero Coins or 60 Rare soulstones for 2400 Hero Coins.
While buying Rare soulstones gets you your hero faster, you should always wait and buy 60 Elite soulstones. By doing this you are saving your hero coins, because 1 Elite hero = 9 Rare heroes and by buying just Rare soulstones you are spending 3600 Hero Coins more than by buying Elite soulstones (that's over 1 Rare hero difference!).
You can also buy 60 Rare soulstones in the Labyrinth store for 2400 Labyrinth tokens.
During your climb in King's Tower, you can win a decent amount of Rare soulstones and for beating some specific levels (e.g. multiplies of 10) you can get Elite soulstones.
While the reward for a single floor isn't massive you will be often conquering multiple of floors in a row earning substantial number of soulstones.
Players can also get 5 Rare soulstones every day and 10 of Elite ones every week. To get them you need to do your daily and weekly quest and you will receive soulstones among other items as rewards.
Both types of soulstones can be a reward in the Arcane Labyrinth. You can get them after beating floor bosses from the reward chest.
Daily free gift and Deals
You can get 2 Elite soulstones and 20 Diamonds every day for free in the Gift shop.
Additionally, you can buy more of them as gift packs. Daily 0.99$ gift gives you 10 of Elite soulstones, which is one of the best offers worth-wise in the whole Gift shop.