A leaf spring lift is rated for approx. 3yrs. After that they start to sag, and the cheaper ones seem to go faster than the more expensive lifts. The age old "you get what you pay for" comes to mind.
Back to your question, you will probably pay almost as much for a pair of 3.5" lift springs as you would for an entire lift kit.
If the Jeep is sagging more on 1 end than the other and you think that the leaf springs are the cause of it. Then the best thing to do is to replace the entire lift kit.
I have seen guys that go the route of extended shackles on the end that is sagging, but if its the front and you run extended shackles, it puts the steering geometry out of whack.
'87 YJ, 4.2L built and bored, 8.5" of lift, 8.8 rear, AX15/NP 231, Tom Woods drive shafts, Flat fenders, 12,000LB winch, Custom bumpers, Rattle can paint job. Never completed...