Hi Peter and welcome to the forum,
It is an issue for sure and there really isn't many options around it. You could of course have everything sent to you firstly, then repackage and on-send as one package, but of course you have extra shipping fees involved and you can't send until the last of the order arrives, so that could well be a longer delay for your customer.
I think the best option is to use multiple suppliers if you find more than one you want to work with. Of course each supplier may offer different shipping times, so as long as you provide that information within the relevant listings, the customer will be aware that different purchases have different shipping times.
You can also reinforce that to them at the time they place the order, just to ensure their expectations don't exceed the reality of the order.