The keys on the keyboard are all uneven, as in some sit higher than others. Is there an easy way to get them straightened out?

Probably missing the little rubber bump pads under the keys.
They are part of the "guide" that is the upward tab on the frame.
If U remove the spring U can get the key off the frame.
Can't remember exactly, but just give it a good look and I think you
will find the solution.

Where can I get those rubber bumps, or what can I use as a replacement?

I can't recall what they look like. It was sooooooooooooo looooooong ago

try disassembling a few keys. U said "some", so that means "some other" keys are OK(?). Starting at an end key is the easiest. Determine where U can disassemble the least number of keys to get at least one good and one bad. Once you see underneath a good AND a bad key, U will probably think of a solution right away. the "rubber" fits over the vertical "tab" and acts as a guide from left to right as the key is pushed. there is another rubber thingie at the back of the key so that when U release it, it doesn't go klunk on the steel frame when the spring pulls the back down and lifts the front of the key back to normal resting position. I'm pulling this info deep out of my subconscience, because it's been soooooooooo looooooooong since I looked under a keyboard.