Where should modal content be placed?
There are couple of things you could consider before deciding on a particular approach. 


Is this data static? 


If yes, then you could store data in memory and retrieve as needed. Performance should no longer be an issue as our js libraries are being loaded on demand and I am sure there are many other ways to optimize your code to work for your particular need.


Is this dynamic data stored in db?


You could expose your data using the built in REST solution. Once you have your data exposed. You could just use jQuery REST api's to retrive your data. 

Hope this helps.