The short answer is no. For web design agency/freelance to come up with mock-up, we need to understand what you want to achieve with the website, what you want to do and how are you going to get that, because if we provide you with a mock-up but we do not understand anything about your business and how you are going to use this website for. The free website mock-up will be pointless, because there are many website templates sites you can get ideas for references, such as TemplatesMonster, Themeforest, Awwwards, etc
From website design company management point-of-view, when the mock-up of the website is completed, the project will be considered already get done about one third or half. A design mock-up is expensive, just like you are not going to any gallery and ask for painting at your house for after a month, then only deciding whether want to buy or not, and this is not going to happen. Hence, similarly to web design mock-up, there is small fee to pay rather request for free, then later you decide whether like it or not and to continue with website development phase.