This doesn't mean you need immediate replacement though. I know of many cars where the owner has adapted by learning doubleclutching to avoid the expense of a rebuild. Bobbing around like an old caddie isn't the best nor safest way down the road. Phil Kalamaros advised me this is a shock issue. Shocks in any car can 'go' after as little as 20k miles.
Springs are supposed to hold their rate almost indefinately - the issue with old springs is sag.
There are many sources for suspension bits and here you've got to make up your mind if you'll be racing or not, since swapping springs and the ensuing alignment job are not something you want to repeat often. You can go Bilstein or other as well. If you do go used, I am wondering how you can be assured about condition unless the seller has a shock and spring dyno, in the least you should be able to take the used components to a race shop that does for verification prior to installing them. Plus you get full support from the very respected Devek shop. Can you say go kart.