If you need a shuttle are you sure about the model #?? all the model 15 take a regular looking bobbin. does your bobbin look like a traditional bobbin or an enlongated spool? If the latter then I don't think you have a model 15 but maybe something else? I am not an expert though. Just know a bit about mine.
Here are two links to antique parts. The first one has the old fasioned bullet shaped shuttle but says it is for different models 27, 127 , 28 and 128. My treadle took the shuttle from this site but i have a model 27 from 1901.
http://sewingmachine221sale.bizland....re/page47.html
If you scroll a ways down they show bobbins for the model 15.
This second URL has parts for model 15s you just have to scroll down a bit.
http://www.sewingpartsonline.com/sin...odel-list.aspx
This site says to contact them if you can't find what you are looking for.