Where can I buy a straight six cylinder engine 250c.i. for 69 mustang.

S&J engines, EBay, Summit Racing use to have them. You might find a good rebuildable one in a wrecking yard.

Over recent years, I've found that owners' clubs are a great source for parts. Often you find people who have them left over from rebuilds or have good information about sources; especially local sources. Look for Ford or Mustang owners' clubs near you or go to Hemmings Motor News..... BTW, that original six banger is a rare find now. If the car's in good shape you might consider a new crate engine; maybe a modern four-banger. It has more power and torque than the original six and you won't have to worry about a rebuild.

thank you for your answer kind of you