This is off a Savage 99 358. F model I believe. The sight wont stay up. Spring broke I guess. Can I fix it? If not, where can I get a replacement?

It's not original anyhow, so just buy a replacement for less than $20. I've seen them on eBay among other places.

What would be the correct one?

I think the 99F swtiched to the fold down sight in 1959. So what is correct depends on what year your rifle is. What is the lever boss code on it (oval stamp on bottom forward of the lever hinge pin will have 1 or 2 digits followed by a number).?

fold down sight for 99F that started to be used in 1959:
2nd is the fold down used even later.

It could be original depending on the SN.

It might be a Lyman or Marbles. Is it marked?

You can get a Lyman 16C for a decent price.

http://www.opticsplanet.com/lyman-no-16-folding-leaf-sight.html