It may be as simple as uneven pad deposits on the rotors.
They would be visible as slight discolorations against the surrounding metal and can be removed using garnet paper, which you can find at any hardware store. Avoid using regular sand paper as it will actually make any deposits (and corresponding vibration) worse.
Worth taking a look at since it's the cheapest potential fix.
Regardless, you need matching tires. I've got a pair of Bridgestone's laying around doing nothing. What model and size do you have on your car? If they match up, they're yours for the cost of shipping - let me know.
