So if you wear a 40 suit coat, you will probably be in a 50-ish Dainese. How broad are your shoulders. I find that that tends to be a minor issue between sizes, but sleeves can be critical. If you wear full gauntlet gloves you have some wiggle room, but if wearing motard style gloves you don't want a lot of exposed wrist.
I think you will be fine with the 48. If you want a bit more body room you could go up to the 50....
As far as ordering, I would go with Salt Lake Motorsports. They have a full catalog online and are very helpful over the phone. I purchased some casual stuff from them and had to exchange an item and they made it very easy. The price will be retail, but you won't have sales tax to deal with...so that can offset shipping some.