There are two types of replacement springs around but not sure if your going to find anyone with them sitting in their garage.
Esplier were one, these were suppose to be a good fit for the Q45 with the V8 weight, the Tein springs I put on my 97' about 3-4 years ago and they have performed very well IMHO. I got them from a guy that got banished from this board but I'll bet he can still get them. Send me an e-mail if you want me to see if he can still get them (
[email protected]).
You should consider replacing the shocks/struts if your going to do the lowering springs. There is a shop that can order those but I hear it takes several weeks, you might have to settle for the stockers from IOS.
Worst case order the whole set of new springs and struts from IOS.