I've got a 239 DAK .40 SAS. Saw someone mention in another thread that parts are slowly going away.

Nothing is wrong with the pistol and round count is probably below 1,000. Just wondering what I should put on the shelf as a backup supply for wear items or parts most likely to break or need replacing...

Springs

It depends on how much you shoot the gun. Odds are that you will never need any parts.

Find the sig kit of parts and get a couple.

I have .40 and .357 Sig P239s and picked up the spring kits, another set of the hard to find original grips (which were the really nice light wood grain) and most of all... magazines.

I picked up a few OEM sig mags just as Sig announced the ceasing of the P239 line. At the time I was finding OEM mags for $22 to $25 dollars. In the short bit of time since they have significantly climbed in price.
(Take a look on Ebay). I wondered if since the P239 was discontinued Mec-Gar would be getting rid of any P239 mags.

So sent a note to Mec-Gar and got a nice response. "Those magazines are made exclusively for Sig. Our agreement will not permit us to sell them aftermarket".

Turns out that Mec-Gar is the Original Equipment Manufacturer (OEM) of a variety of popular factory magazines for brands including Beretta, FN Browning, Charles Daly, Colt, CZ, Kel-Tec, Kimber, Remington, Smith & Wesson, STI, Taurus & Walther.

I suspect magazines will be the main challenge in the future for P239 parts.