I don't know of any place you can find a pre-made bezel like the Happ ones for a 21" computer monitor. I was in a similar situation a while back...
I had to trim a Happ 19" bezel for a 20" TV, which should be pretty close to a 21" computer monitor. Your computer monitor doesn't happen to be one of those true-flat screens with the curvature on the inside of the tube? A Happ bezel won't fit one of those very well because it is formed for a typical tube curvature.
The stock opening on a Happ 19" is right about 18 5/8". The red outline below is about where I had to trim the Happ bezel. If you are careful with a razor knife, you should get pretty good results. I took my time and ended up making multiple cuts until it fit well.
(The stock bezel pic originally came from dhansen!

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