Skill Focus Item?

[undead hero]

Where can I find the rule for items that boost skills?

Edit: creating said items, like a +20 Concentration Item

[hydraa]

the section on creating magic items in SRD or DMG

Bonus squared × 100 gp

[Yuki Akuma]

On the page titled Creating Magic Items (http://www.d20srd.org/srd/magicItems/creatingMagicItems.htm). Second table.

These are only guidelines, though, not hard-and-fast rules. Hence the "Estimating" part of the table's title.