Yes I would recommend you this list of yoga retreat here you can get a comprehensive list of retreat centers in India , you can visit any of them as per your convenience If you found this list useful , kindly share your views.
Vijay sing (health and yoga) . , I checked out your link and am not impressed. It is simply some "middle man" making a lot of money from unknowing foreigners who have never been to India before.
most ashrams in Rishikesh charge a fee or its donation. you can easily take the train from delhi to rishikesh.
I do not recommend pre-booking any "retreat" or for that matter any tour or group thing from outside of India or from within India either. I have seen how this works in India.
If you google rishikesh and ashrams you will easily find a list of at least 12 ashrams there but surely there are many more you will find once you get there.