You can buy them at Homedepot/Lowes in the outdoor gardening area. They come in huge spool, I believed it's something like 4 feet tall and 4 feet wide. It's like $28.99 per spool. I remember seeing them when I was looking mesh myself months ago. They come in clear and black.
I plan on using those Finnex rim brackets from eBay, so the mesh has to be sturdy and not so much like netting. And 30 bucks is a little steep for a simple DIY lid, especially since I don't need that much mesh to cover a 10 gal. I will be going to Home Depot soon to get some other stuff, so I'll check out that mesh anyway. Any other ideas?
It shouldn't, the holes are big enough to let light pass through it. Some people use window screen that is then made to fit the window edge holder and they use that as a lid to keep fish/shrimps from jumping out the tank. That route, it does block some light. If you don't like metal or aluminum grids... Homedepot and Lowes carry those kitchen light grids. They are made of plastic and is white in color. They are called Egg Crate Lighting Panel.
I would recommend egg crate. It won't rust, has no sharp corners, is designed for the purpose of letting light through, and is about $12. Lowe's, HD, Menards etc. will all carry it, either in the fluorescent lighting section or ceiling section or both places.