I could't be bothered trying to find every orb, so I decided to farm them from enemies. I found that the two best farming levels are level 13 and level 14. It turns out that you can get 30,000 every time you play level 14. However, every time you play level 13, you can get between 70,000 and 80,000 orbs per play by defeating all the enemies at the end. Each time you play the level, it takes about 20 minutes.
I managed to get enough orbs looking for fairly obvious orbs and slightly hidden orbs on a regular playthrough of the game that I only had to replay level 13 twice to get enough orbs to buy everything. Hope this helps.
Edit: Apparently, there may have been an update limiting the number of orbs you can collect since I first posted it. I have no way to confirm this as I no longer have the game. However, I still imagine this remains the best level to farm for orbs.