while it is kind of apparent the person's in this thread who know how to make a sugar free fondant, will only share for finacial gain, which is sad - i signed up just to share some experimenting iv been doing in trying to re-create icing, and yes trial and error it is, but if you find a way i do believe one should share it.
While i havent created fondant as its not something iv required lately, i have made other frosting and hard icings sugar free AND low carb ( i do low-carb dieting which is why im after sugar free). i use sugar free confectionary powder which has so far worked for icing, glaze and other frostings - so i dont know why it wouldnt work with fondant.
I would especially recommend using a bit of the swerve confectionary powder mixed into a 50/50 ratio, and perhaps melt the swerve sweetener aswell as it contains Erythitol which is the digestive friendly sugar alcahol, and provides "bulk", It also has the ability to caramelize, unlike standard sweeteners on their own. For me personally as i like low carb everything i have done a mix of LC foods carb free Vanilla Frosting,, mixed with melted butter or coconut oil. I also found that the less liquid added the icing tends to harden which is good, and why i think it would work well for sugar free fondant.
I also add some Carb free milk powder and extra cocoa butter to make a low carb white chocolate - i would like to make a fudge which is more "fondant" like as i like my stuff sickly sweet (so i only need a small amount) but to do this im going to attempt a fondant base while adding a caramel flavour (with a bit of peanut butter and SF Caramel sauce) So i think if i can create this, then the LC foods Vanilla Frosting, mixed with Swerve confectionary sugar would be perfect for your requirements.
Although if your creating cakes for re-sale you'd prob have to charge your customers alot more as it sure isnt cheap :( - i dont know if i can post links here, but google LC Foods Vanilla frosting, and select the second option on a website called hold the carbs. they also sell other useful cake products such as carb free flours, carb free condensed milk, stevia, monk fruit sweetener, Powdered erythitol and their sugar free confectionary powder. its just a matter of trial and error, and mix and match until you create what you need. A bit of advice is use a variance of sweeteners in your products, if you use the same sweetener you will find the aftertaste ovious, but if you do a mix of several sweeteners / or several different brands, the aftertaste of any sweeteners is not apparent.
Also if your after a cheaper option, and your not afraid to use sugar alcahols in your icing - (like they do in making SF lollies etc) then the cheaper sugar alcahols are your ticket - products such as malitol (apparently still raises BS levels), sorbitol not so much, Isomalt is the better option of all the availabilities - these are some of the sgar alcahols used that dont have an aftertaste - the prob with erythitol is it has a sort of "cooling" effect, which is why it needs to be mixed with other stuff to lessen this effect. Most of these products can be purchase through an online store called netrition. have fun :)