it's hard to find someone qualified. I've been very lucky; one of my friends is a linguist, and she has been a wonderful help with grammar and structure. Two other friends were both in the Navy and did PR work, one actually wrote for a Naval publication. And fortunately, they are honest with me if they feel I need to make changes in how I structured something or if something doesn't make sense.
I also joined a local fiction writers group with several published authors, poets, and playwrights as members. I haven't been attending meetings for long, but they were a real help in getting my query letter in better shape. The letter still needs work but their suggestions really helped me focus.
Perhaps checking with a local college or bookstore might yield someone who would be willing to either review your manuscript for free or charge a nominal fee.
I'm not as far along as you are, but I'm as much in the dark as you are about where to turn. However, congratulations on getting an agent interested in your work. Good luck!!!