@nacordiaz according to the information you are giving us, yours is an iMac14,3 Late 2013. Take a look at the board itself. There might be a 820-XXXX number on it. That is the number you want to use for an online search. You model uses amongst others a 820-3588-A board which is a 2.7GHz Core i5 with integrated CPU It has a non-removable CPU and GPU. Apple part number for that is 661-7923. Other boards for your iMac are a 2.9GHz (Apple part number 661-7503)and a 3.1GHz and have a removable CPU option.
With those numbers you can most certainly match the board in your iMac and purchase wherever you would like. There are great places like this one and many more. It’ll depend on where you are located at and whatever seller you are comfortable with.