Wow, THANK YOU ALL for your replies! Now I'm wishing I had the budget to try every suggestion in this thread!
This will probably sound dumb, but I've only ever tried a couple juices that said "custard" on them and maybe this is my imagination, but they seem to taste different than the few I've tried that don't mention the word custard. So I'm wondering - is there a sort of general difference between something labeled "vanilla custard" as opposed to something labeled just "vanilla" or "vanilla cream" ?
In the meantime looks like I need to take a look at each of the suggested pre-made ones in this thread. So many choices... and some really do sound amazing.
Thanks everyone, I wasn't expecting such a number of great replies & info because I figured "ah this question has probably been asked so many times in the past I probably won't see more than a couple replies". I was way wrong, and you guys rock!