not necessarily stiffer springs on those, but they are progressive springs and they are generally crap. but for spring rates check out this post
http://www.ducatimonsterforum.org/index.php?topic=39242.0best option, and possibly even cheaper than the auctions u are looking at there is to simply replace the spring on your existing shock unit. you'll have to do your own research but i imagine you would get a new spring at the correct weight for you and your bike with your riding conditions for about $100. traxxion and gpsuspension come to mind first for me.
if u want to upgrade the shock unit there are many latter model monster and sbk shock units that will bolt onto your bike, but to get the best from them they will prob need a re-spring too. do a search and you'll find many threads with the options and examples og what ppl have used.
for an example i picked up a 749s (same as 999) 3way adjustable showa with the height adjustable link rod attached for $65. add a spring and i have a rear shock unit that is more than adequate for whatever road use i could possibly throw at it.
good luck, Cheers Clint