Trainerlever 30 is correct so you have to do some leveling up
before you can get MissingNo. However, once you reach that level,
you should check out the
announcement for PH's 3rd anniversary as that was when it was
released and the hints to start the little 'quest' to obtain
MissingNo are found there. Just be sure to carefully read the
"code" provided and it should give you your instructions.
MissingNo isn't tradeable so doing that quest is the only way to
obtain it.
(Additionally, you can get a MissingNo once every 7 days and every
50th MissingNo you obtain is a guaranteed shiny if you'd be
interested in that.)
As to whether there's more glitch mons, the answer is no as far as
I'm aware. :,]