Did others than the 11 Apostles see the risen Jesus?
There is at least one other record that says a group consisting of more than the apostles saw Jesus after his resurrection.
Luke 24:33-37 (ESV quoted):
And they rose that same hour and returned to Jerusalem. And they found the eleven and those who were with them gathered together, saying, "The Lord has risen indeed, and has appeared to Simon!" Then they told what had happened on the road, and how he was known to them in the breaking of the bread.
As they were talking about these things, Jesus himself stood among them, and said to them, "Peace to you!" But they were startled and frightened and thought they saw a spirit.
The size of the group, though, is not described.
John's account of the same incident in John 20:19 suggests who 'those who were with them' were:
On the evening of that day, the first day of the week, the doors being locked where the disciples were for fear of the Jews, Jesus came and stood among them and said to them, "Peace be with you."
The number of disciples is not described here either. I think the soonest estimate of the number of disciples comes in Acts, where, shortly after Jesus' ascension, there were 120 disciples at the time it came to replace Judas:
And in those days Peter stood up in the midst of the disciples (altogether the number of names was about a hundred and twenty)
(Acts 1:15, NKJV quoted - some versions have 'brethren' or 'brothers' instead of 'disciples' with the same meaning)
This is a point against there being 500 who saw Jesus - one wouldn't expect the number of followers to go down after such a miraculous appearance. There are two possible resolutions to this, if we want to take Luke 24 and John 20 as representing the incident that Paul is talking about:
- This may have been a different assembly--a "scene change" is implied by the introduction "in those days"--so it isn't necessary that all of the people would still have been there for the assembly.
- The figure of 120 might refer to men only, and the figure of 500+ might include women and children. (In the story of the feeding of the 5000, for example, we're explicitly told that women and children were not included in the count, so there is precedent for this sort of practice.)
Where is what they wrote?
As for where their writings are (which I suppose is the main part of your question) this probably can't be answered by Scripture alone.
Of course one should probably take into account that possibly not all knew how to write (or write well) and even if they did write, what was written may not have been preserved (for various reasons). And probably not many would have needed to write. It might have been easier for them to tell everyone they knew in person.
Further, not all would necessarily have gone on to a missionary career; as everywhere, there are more followers than leaders in the world. They might have seen and believed, but family concerns or poverty or a poor reputation--and we know many who followed Jesus were poor or of sinful repute--might have kept them from the business of spreading the gospel, so they might not have had reason to do so.
And even beyond this--and probably the largest issue--was the fact of the persecution of the early church. People who preached Christ were accused of blasphemy (e.g. Acts 6:8-14), warned by the Jewish leaders not to speak (e.g. Acts 5:27-28), beaten (e.g. Acts 5:40), imprisoned (e.g. Acts 8:1-3), and put to death (e.g. Acts 7:54-60). The actual persecution would have silenced many; the fear of it most likely would have silenced even more.