Poplar wood was very common in American vehicles, light, wide, cheap. Maybe someone who is familiar with woods could look at your wagon and tell you what was used. Many are made of several kinds of woods for strength, then painted so they appear to match.
You might check in Milwaukee to see what records are available on your carriage maker, Library, Chamber of Commerce, Historical Society are good sources for pinpointing information.
The Carriage Association of America keeps a lot of records on old carriage makers, famous makers. Do know that often every town had a local carriage maker, but there is not any information on them in the CAA's centralized location. We have had at least 5 local makers here, only about 15 miles apart! Same thing in other places. The town where they were located usually is your best source for real details, old newspapers, historical associations. Contact them and ask your questions. Might help or they could give you more ideas of where to search. http://www.caaonline.com/
A good source of parts for buggies, wagons, would be Witmer's Coach Supply. They are located in PA, have always been very helpful to us. Probably getting their catalog would be your best start place, so you can view it to get correct part names, designs, before trying to order. They do seem to have almost anything you might need!
Witmer Coach Shop
1070 W Main St
New Holland,PA 17557
Good luck with your project.