I hear you, I have the same issue.
The problem is, that even if you could go into a shop, it's still not always possible to get the fit right until you really know what you're looking for.
My advice would be to order the two nearest sizes to your guess from a retailer with a good return policy and try both.
The ones that feel perfect.. are probably the wrong size and will be too sloppy one you wear them a few days. You probably want half a size smaller than that.
People will tell you that getting them heat moulded is bad because they pack out too much. But really they shouldn't fit until they've been heat moulded. In many cases they just bought the wrong size.
Your toe should be touching the end of the liner until you stand up in the boot in a snowboard stance, then it should retract slightly.
The liner might feel very snug at this fit but will pack out and form to your foot so this will be temporary.
Also, take the liner completely out and put your bare food in the shell and push it all the way to the front so your toes touch, you want to make sure you have a gap or around 5-10mm between your heel and the boot.
And after all that, some boots just fit some feet better, depending on with of foot and hight of arch and you won't know until you've ridden it.
I've learned this by being on my 3rd pair of boots in 12 months.