Kashmir was certainly a historical center of Naga veneration. But honestly, everywhere I have ever poked into in India, Nepal, or Sri Lanka, I have found strong traditions of Naga veneration, usually not obvious and easy to miss, but thrivingly practiced right there if you go looking for it.
“The Nagas’ favorite places of visitations are the banks of the river Ikshumati, the Naimisha Forest on the shores of the Gomati, the northern banks of the Ganga, and the Nishada land.” - Varaha Purana.
I can certainly confirm the “northern banks of the Ganga” part - the upper reaches of the Ganga in Garhwal are dense Naga country. Manasa Devi is the Kuladevi of many of the Garhwali people, who venerate her as the local Naga Queen. Whole valleys and villages there are named after local Nagarajas, who have many hereditary priesthoods. An interesting point is, this region is not at all famous for Naga veneration - but it’s huge there. I visited multiple villages where the main temple is a Naga temple. This is a recurring pattern I have found - places where Naga veneration is a major thriving tradition are still not known for it, it tends to be quiet and not attract attention.
Other historical centers of Naga veneration were Rajagriha (modern Rajgir in Bihar), Mathura, Takshashila, and all throughout Kerala and Karnataka.
But honestly, pick anywhere in all of South or Southeast Asia, go poking there and I suspect you will find a strong history of Naga veneration, and quite possibly living traditions.
Veneration of Gandharvas and Apsaras is/was popular in Kerala, northwestern Pakistan/eastern Afghanistan, and the Himalayan foothills, particularly Uttarakhand. I have traveled up in the Garhwali foothills a lot and found a lot of local lore of Apsaras as beautiful but volatile forest spirits who inhabit remote lakes, springs, and high meadows there. In the local dialects they don’t usually call them Apsaras, but rather Achharis or Kecharis, but if you ask they will confirm these are called Apsaras in Sanskrit. These communities still offer small ceremonies, incense, and specific foods at high-altitude groves to appease the Achharis, to ensure safe passage through the mountains and to prevent them from causing Achhari Rog (mental distress or infatuation) in young travelers.
In Kerala, there are living traditions of Gandharva veneration. For example in the Theyyam tradition of spiritual dance/channeling, there is a whole subcategory of Gandharvan Theyyam.
The Pulluvar community there also venerate Gandharvas (and also Nagas) in Pulluvan Pattu. Keralites specifically venerate Pala Gandharvan, associated with the fragrant milkwood tree. Especially pregnant women, or women seeking to become pregnant, often do Gandharva veneration practices in rural Kerala still.
Uttarakhand and Kerala are just where I personally have spent the most time in India and encountered these traditions. They probably exist in many other areas as well, which I have not visited or spent enough time in to discover these things.
Ancient sources say that there were strong traditions of Apsara veneration along the ancient Sarasvati River, back when it still flowed.