When you were growing up, there were probably one or more stores near you that did 'alterations'; they would take in the waist of a pair of trousers or expand the chest of a jacket. They're not as common anymore, because more people are buying clothes that fit them properly. That's because there's a demand for well-tailored clothes.
If you don't have the skills to tailor clothes yourself, find someone who does and work in partnership with them. You can be the manager, and the marketeer. We've all bought a suit or work clothes from a high street store before now and been disappointed with the fitting. You could be part of changing that. Once you have a couple of happy customers, you won't even need to worry about the advertising side of the business. Word of mouth will spread around town, and the people will come to you.