In the MIC is an item (amulett IIRC), which allows you to raise you ML for a few manifestations/day, but it does not stack with Overchannel or Wild Surge (it explicitly says so).
The orange ioun stone of the DMG raises the CL of all casting classes by +1. In the XPH is a section about ioun stones and their uses for psions - the orange stone is NOT included. Because of that, we assume the +1 to CL does NOT apply to ML for this item (RAI). Further, a bonus to ML is better than a bonus to CL, thanks to the various augments most powers offer. So, you should NOT assume that every CL+1-item is automatically ML+1.
That being said, a +1 to CL slotless item is 30,000gp (Ioun stone), and slotted items (ring and robe are in the MIC) are 20,000gp. Take these prices by 1,0 or by 1,5 for +1 to ML-items, and you should be fine.
We had a campaign, where the psion was overchanneling often. He received a crystal, which gave him DR 3 for overchannel-damage. Further, the DM allowed him to use a Mystic Ring of Fire (+1 CL for fire spells, +2d6 damage 3/day on a fire spell, 7,500gp) for his manifestations. But dealing fire damage is not the most efficient way to spend you PP, anyway.