Brake Booster 96

I beleve my brake booster is going out. Really hard pedal. Kinda came on slow over the last few weeks. Only hard occasionally. Now its hard all the time. How hard is it to replace it (DIY)? How much? Best place to purchase ($)? is there some way of checking to confirm that this is the problem and maybe not somthing else?

B4 replacing - check that the vac line is clear. It can sometimes plug up at the intake.