I ordered parts from www.mcmaster.com (McMaster-Carr). They have a superb web site. You pretty much describe what you're looking for, and the site presents you with a list of matching parts and additional selection options. Before long you're down to a part or two and you can select the one that suits you. It's painless.
On thing I discovered about my spindle recently is that it's not like the others I've seen on the web. The bearing shoulder on mine is fully 1/2" shorter. That, and the lack of a flange bearing, was why the headstock had that weird thrust path.
Here's the spindle. The part we're concerned with is the shoulder in the middle of the image. I have to get the force from that shoulder to the thrust bearing.
I ordered a remarkably thin needle thrust bearing and a sintered bronze flange bearing. My friend used his Sherline to part the flange bearing to length. In this picture you can see how the bronze bearing channels the thrust from the spindle shoulder to the thrust bearing.
The back gear isn't really secured now that I removed that huge bearing. Originally I was going to secure the gear to the cone pulley but I've nixed that. Instead I turned a UHMW retaining ring that consumes the empty space between the back gear and the thrust bearing. Here's the ring. Note how it rides on the flange bearing. The ring fits relatively loosely and, being UHMW is very slick. If all goes according to plan, the back gear will not be able to affect the bearing.
Here it is all together. The back gear has no where to go.
Now there is still a little work to do on the spindle. The mating hubs of the bull gear and cone pulley are abrading one another. This is a very high-wear part of the design. Perhaps over time a bearing between has been lost. But now the hubs have rubbed down and allowed the outer edges of the parts to touch and scar. I'm going to turn the hubs smooth then remove enough material from each so a 1/8" washer bearing fits between them. I'll leave the bearing a little proud so it prevents the outer edges from touching. This should be easy enough.