Where can you buy a Prismana Orb?

The shop in question is the Windborn stamp shop, so the orb costs stamps rather than anything you can top up. Open the stamp shop, which the game also labels the Custom Shop, and look under its seasonal tab. You get into that zone by spending credits. The same seasonal section also stocks Sparkling Eggs, Sparkling Cubes and Kappa Fruit, so the orb competes with them for the same stamps.

You can also get Prismana orbs without buying one outright. Spending stamps in the shop unlocks milestone rewards, and the 3,500 stamp milestone pays out an appearance pack, Prismana orbs and phenomena crystals. Every stamp you spend in the shop counts toward that total, and that includes stamps spent on other things.

Earning the stamps for a Prismana Orb

Windborn stamps come from the activities listed under Stamp Rush in the Institute Guide. Turning in Aniimo reports and fighting Ultra Aniimo battles are two of them. Sitting in on the Starcross Tea Party at the Astra coffee shop also hands out Windborne Stamps passively, along with up to 150 Voxel Coins a day.

The shop's stock resets every week, and anything you did not buy that week is gone. If an orb is on the shelf and you want it, buy it in that week.

Prismana Petals from the same shop

The stamp shop also sells Irisalis Prismana Petals for 50 stamps each on a weekly rotation. The option is not clearly labelled, so check the seasonal stock carefully if petals are what you are after rather than the orb. The full stock list and best buys are on the Windborn Stamp Shop page linked here.

Not settled: How many Prismana Petals the stamp shop offers each week is not settled. It has been given as up to four, as up to 50, and as random.