"I plan to buy a house on the Costa Brava in Spain. Can I find a U.S. lender for the mortgage I will need?"
No, the best you could do is get a referral to a local (Spanish) lender. With very few exceptions, U.S. mortgage lenders don't lend outside the United States.
In most other countries, you must be licensed to make mortgage loans, which U.S. lenders are not. Even if no license was required, U.S. more...