Hey, I am a machinist for Anheuser-Busch of 20+ years. EZ outs are very common. Sears, Snap-on, so on. A NAPA or some other places should have them. They are tapered left hand threaded units of different sizes and cheap. I recommend this over the slot and screw method, but It's OK if you have no other way. You will also need a center punch and hammer.
If you can get a drill to the broken bolts you are in luck. You will need to smooth off the broken surface so you can punch a hole as close to the center of the broken bolt as possible. This is so you will be drilling into the center of the bolt. If you try to drill it without center punching, the drill could walk towards the side and you will be drilling into the threads in the hole, opening a whole new can of worms. If the bolts are sticking out you can file or gently grind a flat surface on them and take the punch and a small hammer and put a dimple hole in the center for the drill to use as a guide so not to walk to the sides. Use a drill size as large as possible but so it will not get into the threads. Use a new drill and go with patience. Take your time, keep it straight and don't push on the drill too hard. DON'T GET IN A HURRY. Let it do the cutting, not your arm pressure. If the bolt holes do not have a bottom and you can see the other end of the bolt on the other side, drill all the way through. Sometimes the drill will catch on it and thread it on through. Insert an EZ out that will go in about half way or so and gently tap it in with a small hammer. Put a wrench on the end and try to back it out. If it slips or strips. Tap it in as you turn it or try a smaller or larger EZ out.

Good Luck.