I've been reading the js-stellar-sdk and I didn't find a way to retrieve the operations for a certain Asset.
How should be this done?
You can do this with two ways.
You can scrape and traverse through all operations and save records with your asset into your own database (I'm using this way before I build stellar-core horizon instance). You can use this method if your asset is still low operation volume, because horizon is limit rest api with 3600 request per hour.
You can build your own stellar-core horizon instance, this is more flexible way because you had all data you need. I suggest using docker image (quickstart) to start.