I agree with Rich's advice. A steroid injection, if successful (they aren't always, and it's a bloody painful shot), will relieve the pain, but if the problem is a biomechanical one you'll get further inflammation and pain or, worse, the plantar fascia will scar and shorten, leaving you with permanent foot problems.
Deep tissue ultrasound has some, but limited, effect statistically, but is worth a try - you may be one of those for whom it does work well.
Ultimately it would be worth having a gait assessment. Shoes are important, but the underlying problem could be as far away as your lower back.
(Posties get it quite a lot. It used to be called "policeman's heel" because coppers on the beat were prone to it. They all ride horses around here nowadays.)