should I suspend a floor-standing turntable?

Put a few very heavy concrete paving slabs stacked on a sturdy table.
Optionally a slab of maple.

Go ahead and mount it on the wall. Most wall mounts are secured by 4 screws that go into the studs. A little puddy and paint will fix the wall "if" your landlord finds out and insists that you take it down.

Unfortunately I live in Southern California. Thank you for the offer though Dorkwad.

and @Brf I live with my parents (I'm a student) and I don't think they'd let me do the mounting, hah.

At this point, I'm considering just buying a block of maplewood or granite and somehow affixing it the floor. My desk is quite wobby as well, and there's not really a place to put a turntable. Does anyone have suggestions for places to acquire such wood blocks for cheap?

Although these are fairly expensive, they illustrate the kind of arrangement I would suggest. A maple block on top of rubber/cork footers. I think you might be able to find similar material for the footers less expensively at Home Depot, or via Amazon.com (search for "vibration damping pads"). Or you can buy the "Isoblock" footers from Mapleshade separately for $24, and obtain the maple block elsewhere.

Regards,
-- Al

There is no real solution to your problem. Soft things like wood, rubber, etc. will just rob you of dynamics. About the only thing that is realistic, I think, is a three inch thick piece of granite, not marble which just rings like a bell. Place the granite into a larger wooden box that is partially full of sand. This will dampen the ringing of the granite. My wooden box had a sheet metal bottom and had no projecting screw heads or feet, which would ruin what you were seeking. Mass and the irregularities of sand were where you were placing your bet.

This does nothing for springy floors. Nothing other than an adjustable pole placed under your floor and run up tight against the floor from below. I doubt if you can do this. Wall mounting is no real solution as the walls vibrate also. Perhaps using the rafters above might be useful, but I've never tried that.