Flight
If flying out of Southern California you can find affordable and direct flights from Tijuana's International Airport to Leon's International Airport (Bajío). If choosing this route you will need to purchase a California Boarder Xpress (CBX) ticket in order to cross from San Diego via the CBX bridge directly into the Tijuana Airport. This ticket can be purchased in advance or the day of at the gate. CBX provides convenient parking located adjacent to the entry point if driving there. Here is the link with additional information https://www.crossborderxpress.com/
Travel Note
Once you land you will need to take a cab ride (approximately an hour and and twenty minutes) to San Miguel de Allende. Cab reservations are made at the airport. Renting a car is not advised as parking is limited and San Miguel is most conveniently explored on foot or via taxi.