Where do you find editors to check your work for grammar and sentence structure?

You're not going to find an editor who will do this without getting paid. The best way to get critique is to use a critique group - see related question - or get yourself a good group of advance readers - also related question.

You can also get conned into paying for "editing" if you're not careful - any agent who charges a "reading fee" is an agent to avoid. A reputable agent is willing to put in the time to look at a new manuscript, because they know that if it sells, they will get a commission!