Search - website spell checker - and you'll find a variety of choices; free and paid. Paid is usually worth the money in most things. Can't make any suggestions. We spell check pre-upload with our own software so no need for a service; plus don't have "by the user" issues where grammar,general literacy, and basic ability to communicate are all bigger issues than spelling. Bigger concern than spell check is grammar check. This requires eyes-on proofing. Valid words will let bad grammar go through. Have never found a substitute for a second pair of eyes.

Also you need to be sure that spell checking is using the dialect of your target audience. You will probably also need an option to load up brand names and technical terms.