For DIY concentrates, I order mostly from
Chefs Flavours in the UK, and also from
Cloudhouse in Germany. I have also used
Pink Mule in Spain for other items, and they carry concentrates. These
three I can vouche for in terms of reliability, if not speed. Chefs is the best, and has a lot of great one-shot flavors (all in one mixes) as well as a huge selection of single flavors. The site is difficult to search, and I find it easiest to use google if I know what I'm looking for. Looking for custard, for example, search this string:
site:chefsflavours.co.uk custard
or with quotation marks for an exact match
site:chefsflavours.co.uk "welsh custard"
I'm not sure what you mean when you ask about "a good - ice- liquid for diy," but Flavor West makes a flavor called
Extreme Ice that's
available at Chefs. If you read the
reviews at the Flavor West site I linked, you'll see you need to be very careful to only use a very small amount, because it's apparently
super strong.
If by "ice" you mean more of a frozen
dessert sort of thing like a "lemon ice," etc., searching for "sherbet" will give you some options to check out.