You won't find any.. the spacers have to be a minimum thickness to be able to machine the counterbore/countersink to enable them to be bolted separately to the hubs (needs at least enough material for depth of bolt head).. As you suggest I'm sure the H&R minimum thickness is 20mm, other manufacturers like Eibach however have a minimum of 25mm..
As long as they are (hubcentric) have a bore machined in the back of the spacer to house the standard hub spigot and the spacer has a spigot for the wheel to locate on then most of the load will be transferred into the hub rather than the wheel bolts.
There shouldn't be an issue with the ones you suggested initially, Just check the length of the bolt has enough thread engagement into the hub, and doesn't stick out the back or it could hit the upright..
Regarding the bolt length @40mm
If it needs to engage 9mm into hub and the spacer is 16mm thick... you are left with 15mm for the wheel thickness from bottom of bolt hole chamfer to the wheel mount face.... what do your wheels measure?
Most of the time your just paying for the name with H&R's and Eibach (especially if your using the smaller width spacers, which are just turned aluminium billet)
