Who sells?

This guy used to sell them, but he still has some goodies nobody else sells... http://juicemotoparts.com/products.html?limit=all

Other places to get them are from online or even some local bearing shops, the size you need is 14x10x16 if I remember right and any of the better namebrand ones will be a LOT better than the stock one qualitywise.
One of the first things I swapped out on my engines before even running them.
The SKF sealed bearings are best for the crank and clutch shafts, and they're the same size, just remove the inside facing seal for crank use and keep both seals intact for the clutch... Both my crank and clutch bearings were in poor shape after about an hour's run time on my first engine so these should be changed out as soon as you can, and especially if adding performance to the engine is intended. Some engines like Neil's or Dax's may already have the good bearings installed but most of them just come with the cheapest ones they can assemble the engine with.

Several chain saw bearings interchange, as do a few mopeds. Check the search box as there are threads on this.

Gilardoni, Tomos, Stihl, Husqvarna are a few that come to mind.
The main improvement is getting a wider bearing that won't move around on the pin so much. The stocker is too narrow.

yes... definitely get the widest bearing that fits the wrist pin... If I remember right there's 17mm of space inside the piston and the stock bearing is only 14mm wide so there is a lot of side play in that area.
I also found wrist pins I think for Stihl chainsaw engines that are the right size for these engines and there are some chainsaw pistons that are 47mm and use a single 1.5mm ring, but the ring was chrome and may lead to seating problems in our nikasil plated bores. I want to say Motobecane had an engine with a 47mm bore and a 1mm chrome single ring too but same problem, no cast iron ring like our cylinders would do best with.

Another place that comes to mind for wrist pins and bearings is http://www.treatland.tv/, they also sell all kinds of top end kits and pipes for hopping up mopeds and there are several of these kits that can bolt up to the China Doll bottom with minimal mods if you want to build something Really fast. The Puch Hi Hi head is also a good alternative to the Fred head and performs really well while improving cooling on these immensly... Here's a link to look around a bit... https://www.treatland.tv/moped-kits-s/55.htm ... A little off topic, but I'm sure ya won't mind...

No problem, even on a stock build where performance isn't as important as reliability, replacing the wrist pin, crank, and clutch bearings with quality units will definitely extend the life of these engines. I know my first teardown of one with maybe a few hours of runtime showed no problems with the cylinder, piston, head, or any of the rotating assembly, but the clutch bearings were already rough spinning and one of the crank bearings was already loose. The wrist pin was still good but being too narrow I changed it out too.
No telling how long that engine would have run like that, but good bearings are cheap insurance.