Bubble on Tire

What is your question?

I have 4 tires with lots of wear,,,,however I DEVELOPED a bubble on side wall....and that tire is no longer available. My question relates to the tire tread design.... Do I need to be concerned with the tread design? Should I replace one or two tires? How would I even go about that? Can I just buy one or two good tires and not concern myself about tread design? Should I put new tires on the front or back to start,,,,,and just rotate them as I normally would? Thanks for your time. Charlie