I'm unaware of any aftermarket evaps for 850s and although there are 850 owners on their 3rd evap, the latest "cure" by Volvo that arrived on the scene about a year ago should solve the problem, that is IF you're willing to pay the price as it's not cheap. Believe it or not, given the choice of replacing the evap only vs the new kit, most people opt for the cheaper evap only replacement taking the gamble that it too may fail again.
The new kit consists of an evap, new accumulator (absorbs moisture as old one becomes hopelessly saturated when exposed to the atmosphere, left opened for more than 20 minutes), blower motor brush air restrictor tape (yes) and a pollen filter kit. I believe the kit's about $250 more than the OEM S/V70 type evaporator but that's what Volvo believes should preserve the life of your 850 evap.
As for other brands, other materials, if you were able to find one you won't really know how well it will perform (do its job of absorbing heat) unless you've seen any proven test results. Considering the labor involved, it's presently a gamble IMHO.