Supplying power to PCB with USB voltage (2.5V) regulator
After clarification from OP I understand that the supply voltage is from a USB, and it needs to be regulated down to 2.5V.

If this is the case, then it is easy enough. You can simply go to an online retailer such as Digikey, Farnell, Mouser or any of the others and look through their voltage regulator section. 

They have parameter searches, so if you want a particular package due to space constraints, then you can select that. You can then view results based on lowest price first if cost is an issue. 

If you know the current your load will be drawing and you know any other specifications that may be important, then put those in as well. The search results will show you all the available ICs that fit your specs.

If you don't have any constraints, and it is a simple enough project, then any old 2.5V regulator that accepts a 5V input will do. If you do have a pretty heavy load, then be sure to check the thing won't heat up too much!