For a bulk buy of small super magnets I ran a search on eBay for 3mmx 8mm magnets and purchased a A lot of 150 magnets at a very good price from a dealer in the Far East with a good feedback. Shipping was free and reasonably fast.
In most yard tracks I locate uncouplers near the clearance point of the turnout to maximize track capacity. On industrial sidings magnets are located to position the cut of cars under a tipple or at a loading dock platform. A couple of my sidings have more than one industry and they have more than one magnet.
For stiffer draft gear springs I used ones I had laying around in my parts bin - some from old Athearn trucks, others from Atlas or Weaver couplets. I haven't searched for a new supply of springs but when I do I'll start by looking for steel ones (Kadee' s aren't steel) close to the dimension of the Kadee ones. I've found that it doesn't take too much added stiffness to immobilize the Kadee delayed action feature.