10k is short hand for 10kg/mm. To convert to pounds, multiply by 56 to get lbs/in.

Some of the stiffest replacement springs are RSR Race, which are 5k front 4.5k rear. A company called KGMM makes stiffer springs, but they are really hard to come by.

Of course you can go higher with coilovers. The popular Tein HE's are 8k/6k. Increasingly popular Silk Roads go 9k/7k or higher. The pro D1 drivers actually run lower rates, around 7k/5k or 7k/6k. All of these rate that I have quoted are for S13.