Jason, I've often had a search on the net for springs after exactly the same frustrations as you.
The trouble is, there are loads of different attributes of a spring such as:
- wire gauge
- length
- internal diameter
- external diameter
- compression force required
... the list goes on, and means one spring that
looks like another might actually work completely differently. Having said that, how technical does it have to be? As long as it fits inside the valve assembly and pushes the valve shut, and allows you to operate the tool, who cares if it isn't the exact one.
It seems that you can only either buy a lorry load from a manufacturer or nothing, no inbetween. So we're kind of shafted on that front

However after thinking this much I usually cave in and just order a repair kit

It's good to change the valve stem frequently anyway as it wears and so do the O rings, might as well use the new stems rather than keep a pile of them in a box.