[HELP] How to display themes preview on demo.mydomain.com

Hi,
My question is very straight forward to those coders who have done it already on WordPress.

I have a section in my site where clients can select any theme and I’ll create a website for them on that theme.
Currently I am forwarding the users to see the demo of those themes on website of themes provider (WooThemes, Press75 etc), but, now I want to display the demo of all those themes on my own domain/website (I have more than 150 themes). I want to do that without purchasing/adding domains for each DEMO samples, I would like to be showcased under FRAMES from my main site using demo.mydomain.com/themename format.

Examples :
main page = https://www.elegantthemes.com/gallery/nimble/
DEMO page = https://www.elegantthemes.com/demo/?theme=Nimble

A detailed explanation is highly appreciated.

Thanks,
OvaisAlam