Looking for a source of vibration dampening sheets

Mike Powell of Verastarr.com sells what you ask about. His product is named Starrdamp, and one side of it is copper which also shields from RFI.
He is currently having a closeout on his mass loading granite blocks which are another aspect of tuning and clarifying.
I have used both of his products in a few of my systems with fine audible improvement.
Herbie's Audio Lab also has great products which you can read about on Herbiesaudiolab.com. I use in a few of my systems and the product line is extremely broad, and addictive.
Both sites list phone contact numbers to speak with the owner. ("Herbie" is a nom de plume) Both are dedicated audiophiles who spend time with a caller.

Mijknarf, A very tiny cost further upgrade to your cones would be to add dampening using herbiesaudiolab.com Grungebuster Dot between the top of each cone and the flat component chassis undersurface. For a microscopic price, there will be more clearly audible upgrade.
Also, sometimes, placing Audiopoints APCD2 Coupling Disc to receive the sharp point of the cone increases the sonic benefit due to a proprietary design to sink the vibration energy even though the point is no longer directly contacting the underlying shelf.
If you call the telephone number of Herbie's Audio Lab you can get an entire course in how to dampen everything in sight since there is more to know than what I briefly touched upon.