RStudio doesn't offer a product called "RStudio Premium", so some clarification as to exactly what you're hoping to purchase is needed.
If you're looking to just get on to a higher tier on shinyapps.io, you can do you at https://www.shinyapps.io/ >> Pricing. There are a number of options listed there, including Starter, Basic, Standard, Professional.
If you're looking to purchase one of RStudio's professional products, like RStudio Server Pro, Connect Package Manager, or Team, a good first step is to reach out to sales, at sales@rstudio.com.