Sorry I am not too sure what you mean by a platform to buy these tools. If you want ahref, semrush or moz then you just visit these sites and subscribe to them. There is no magic platform to gain access to these services.
However, as you also say you are new to SEO, then it's more than possible these tools may be beyond your understanding to get much out of them.
There are many free tools out there to get you started, but a lot will depend on what you're trying to achieve. For example, screamingfrog offer you a free version of their site crawling software (up to 500 pages), Google Chrome offers you an audit by pressing F12, GTmetrix offers you a technical breakdown of your site, Ubersuggest offers you a free audit, Google search console shows you how your site is being found plus much more.
As you can see there are many options and a lot I have not mentioned but in order to know which tools you require will depend on what you're trying to measure / achieve.