Plastic tubes of all sizes and diameters, cardboard one, straws, Shungite, quartz, low cost Schumann generators. Springs, wood, concrete slabs, bamboo plates, cork plates, sorbothane all materials i use in my acousticals,mechanicals and electrical embeddings controls experiments.
Cost : peanuts...
The gear choice is relative to your budget and matter less at the end than the way you implement it in your room.
Dont be a passive consumers learn acoustics basic .
Believe me i dont need to upgrade...