Even if you were to gather all the assorted extractor springs that have been listed for use in the various compact 9's (3913 & variants, and the CS9, and not counting the larger framed 2, 3 & 4-digit single stack 9's), they might not be of much use to you in the event your 908 ever requires a new extractor and/or extractor spring.
Determining which spring is required to bring any particular single stack 9 into the recommended extractor tension range (4-7 lbs) requires the use of a force dial gauge ($150 tool), and fitting a new extractor requires a factory bar gauge ($70 GO/NO-GO tool) for fitting (filing) the adjustment pad on a new extractor. This bar gauge checks the dimension between the inside of the extractor hook and the opposite breech face shoulder.
Since S&W is still able to make the 3913TSW (and variants) for existing LE customer agencies (if they can't talk them into buying M&P 9c's, of course
), they're probably going to remain able to perform repairs on them for some years.
Besides, unless you abuse your extractor by loading the chamber directly and letting the slide run forward on a chambered round (instead of loading the chamber from the magazine, as intended), or you're going to leave the chamber loaded (and the extractor spring under constant tension) for 10-20 years, you're probably going to be using that same extractor & extractor spring for at least 10-12K rounds, or at least 10 years, or more. (Unless you just happened to have received one of those seldom occurring "defective parts", of course.)
You plan to shoot that little gun for 10K, 15K, 20K or more rounds?
You might wear out a hammer or drawbar (rounding off the DA hooks on either) within that time. Maybe. Maybe not. Dunno.
I've replaced some extractors and extractor springs in some older 3rd gen 9's, .40's & .45's ... but the only one I remember having to replace in a 3913 was a very early production model, and according to the owner he'd bought it new and had used it for something like more than 12 years for weekend competitions and practice. The hook had finally broken after so many years of hard use. The guy's a real shooting enthusiast. (I also had to replace the hammer & drawbar in his 3193, and one other, due to worn hooks.)
Now, I
would stock up on some recoil & mag springs, for periodic replacement, if you're going to shooting that much.
Putting aside one of the metal guide rod assemblies might be handy (although don't be surprised if the standard one used in the "budget" 908 runs a lot longer than you might give it credit for).
Now, those plastic sights, on the other hand ...
Enjoy responsibly shooting the 908.
Just my thoughts.