Just buy new a arms. They will have new bushings, new balljoints and it will be much easier.
If you still want to just do the bushings... Check out Rockauto or see if MM has any poly bushings that go in stock A arms. For balljoints get Steeda X2 units if you are doing them separately. You will need an alignment.
If you can still find a set, 03/04 Cobra a arms are the best oem ones.
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2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.