Branded versions - (erythritol/stevia mix) I've used are Sukrin, Natvia, and once or twice when it;s been on offer, Truvia. Try Ocado, Morrisons or Tesco for the last 2 brands.
I've tried NKD Living, and they tend to produce good quality stuff. Their xylitol is good, too. I bought Pink Sun brand powdered erythritol, which wasn't as good -- too clumpy.
It's also well worth checking eBay prices as well, as both erythritol and xylitol can sometimes be substantially cheaper over there.
I have a regular Amazon order for NKD Living's Erythritol Gold, but I didn't really get on with their standard erythritol...
And a warning re xylitol - don't buy it if you're have dogs, as even small amounts can be extremely toxic to them and can cause hypos, liver damage, and possibly death.