I have made my own marshmallow fondant many times. I confess that by using the premade ones I have saved a lot of time and money (believe me, in Norway sometimes to make your own costs more than just buying it, specially if you don’t get a good result at the first trial).
For my cakes I use the sugar paste from Irca – an a Italian brand that tastes lovely vanilla and is very good to work with and gives a perfect finish to the cake. For my figurines and very dark colors I use the sugar paste from Satin Ice. Sometimes I also use the fondant we can buy at Cacas (I know, not a very fancy name for a cake tools shop, but it works for Norway as “cake” in Norwegian is “kake” and when pronounced sounds like “caca”…) but this last one is not one of my favorites. Still it is very good to work with when making figurines.
Here is a very good recipe for LMF fondant (marshmallow fondant). If you are very lucky you will manage to make it at the first trial, it wasn’t my case 😉
Keep on trying and you will succeed!