Replacing MacBook Pro (13-inch, Mid 2012) logic board
The only logic boards guaranteed as compatible with the Mid 2012 13" Unibody MacBook Pros are either the 2.5Ghz (core i5) as you have now or the 2.9Ghz (core i7) unit also used for this model.

The cost of replacing these depends on where you're based, where you purchase from, and whether you buy a new, used or refurbished logic board.

Before considering such a replacement you should also consider whether you feel comfortable doing this yourself or not? You can review the MacBook Pro 13" Unibody Mid 2012 Logic Board Replacement page which details the tools required and steps involved. If you feel comfortable doing this, then you can decide whether or not purchasing a replacement logic board is an economical exercise or not.

Without knowing your location, I can't really recommend a retailer to purchase one from. However, most users who undertake this themselves will purchase replacement parts online from places such as eBay and Amazon, or from some specialised online retailers such as iFixit or We Love Macs.