Well, I like spikes for both sprung and unsprung, but you can find a nice assortment of both spikes and rubber feet at Parts Express. You have to check the size, cause they're really sold for speaker cabs. Many plinths have corner pieces. If you're trying to put an insert into 1/2" ply it would have to be pretty small. For example these require a 3/8" hole.
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=240-725There are sets that are fancy and include cabinet savers, or rubber feet.
http://www.parts-express.com/wizards/searchResults.cfm?srchExt=CAT&srchCat=139I've made my own spikes for tables with 6mm threaded inserts already on the table. This was a common size for Japanese tables. I just bought bolts and ground off the head to a spike with a bench grinder. 6mm nuts serve as jam nuts. If you can use 3 that would be better, but many tables would be unstable. 1 in the rear and 2 up front is preferred.
I almost forgot - any spike or foot that uses an insert will only be as stable as the insert. They have to be glued in place so there's no movement while supporting the weight. Wood glue with the part clamped in place for 24 hrs seems to work better than epoxy w/o clamping.
