Where you would purchase the MS licenses depends on how you get your servers. If you are colocating your own servers, you're probably going to need to get your own SPLA agreement. If you are renting dedicated servers then you'll need to get them from your provider.
I deem you need to have a dialog with web hosting company which provide this services and together try to find the way for the solution. Perhaps you will need to keep up with different providers at the same time and compare their prices
You should write to Microsoft for SPLA. It is better for you to own them since you are colocating. Also, see if the DC where you are colocating can assist you with the purchase and procurement. You may need some help here with sizing and capacity planning. Understanding the licensing scheme of Microsoft is not easy. You can end up saving if you know the tricks or you can end up paying more.