3. SMALL ISSUES
First of all, congratulations! You almost have a book to print!!! It will be tempting to send it out. After all, you have already corrected a few misspellings along the way and you've read through this thing a lot of times.
Take the time to have someone go through who is a detail oriented person. Find someone with fresh eyes not really reading the story for content but for grammar, spelling, paragraph flow, and general readability. I write Middle Grade so I also have to think about vocabulary and grade reading level. Your genre may have similar necessities.
Do it your self - ONLY SORT OF
In all reality, you should not be the one to do this. Seriously. You are too close. You can find an inexpensive proofreader on Upwork. It may not be 100% error free, (get what you pay for), but it will still be better for the expense.
I would say you can reduce your cost by saving the person time (you still need to give it to someone else) if you use / purchase software such as grammarly. Grammarly has a free version, but I do not have experience with it. It will do a remarkably good job going through your work, though you may have to do it in smaller sections since it is an online tool. Again, I do not think it replaces a set of human eyes, but it may reduce the cost of that task.
My other advice is to read your entire book OUTLOUD. Seriously. You will be amazed what you discover. Try it just for a chapter or so and I think you will see how it draws out little issues. If you don't want to say what's on the page out loud, it likely means you need to change that snippet of dialogue, description, or narrative.
Pay someone to do it.
I think this is your best option. There are lots of places to hire a proofreader. I've mentioned a few. The person I use I found through networking. I hired someone to proof Jam Sessions. Then, after publication, I decided to make it into an audible book (will discuss this in a later post). The gentleman I worked with identified a couple of things my admittedly very cheap proofreader missed. He was an author of a Hallmark movie (that starred Kenny Rogers!!) and connected me to the lady who proofed his work. She was kind, affordable, and didn't advertise beyond referrals. She has done everything I've taken to print since.
A word about seeking an agent. A manuscript should be in its best form possible before going to an agent. I suppose, you could get by with taking your first few chapters (what most agents request) to a professional editor with the expectation that when your work is picked up the publisher can help you with this final polish. My book twelve Hours was published by a small, Indie Press (Three Ravens) and they did the bulk of the final walk-through.
-3. Coerce, Bribe, or Beg someone to help.
If you know a librarian school marm who loves to correct apostrophes, knows when to use semicolons, and has t-shirts that say things like "They're practicing Their grammar over There" then you won the lottery. Even more if he or she is willing to help. These people are rare gems. If you are writing a single book, perhaps your own memoir, I would suggest this route. However, if you are planning to write more than one I would suggest finding someone to employ. I have seen groups around that do this level of editing also. In my limited experience they are no fun and somewhat hateful as they bicker over the use of the oxford comma and - if you take corrections from different members - may leave you with inconsistencies. I left the one I was in when they spent thirty minutes debating whether a British spelling was appropriate. It is, by the way. They invented the bloody language.