While a shock adsorber technically only refers to the damping element, in practice the entire assembly including spring is commonly referred to as a shock adsorber. I think think you are on the right track with searching terms like "strut", "shock adsorber" and "shock". The key will be finding a market where something with the specs you want are used and produced in large quantities. Based on your link it looks like you have been searching in the ATV market. You might find some good products in the mountain biking market as well but I know things can get really pricey there. Try searching by specific applications that use similar products such as "mini ATV strut" or "rear mountain bike shock".
If you can disassemble them or even get them apart in a manner that you would be able to reconstruct them replacing the springs might be a good way to go and keep costs down.
As far as specs go there are a lot of variables but the main ones I can think of are: spring rate (which may vary along the stroke), preload, damping coefficient (may be different for extension and compression). Any of these may also be adjustable depending on what shocks you have. I'm not surprised you are having difficulty finding specs as I suspect most of the shocks for ATVs have had little testing outside of does the thing work in application (if they are made in china the manufacturer may not even have an idea what the target specs are ... just what the design they copied looked like
) It looks like the ones you are currently using have a progressive spring rate as well as an adjustable preload. Have you reduced the preload to the minimum setting?
Another idea is to change the mechanical advantage of the suspension mechanism. You could move the mounting point of the shock closer to the pivot of the suspension arm. You could also mount the shock so its axis is more perpendicular to the path made by the point where the shock attaches to the suspension arm.
It sounds like the track is putting a lot of perload on your shocks, can you make the tracks longer?