These are probably the least expensive (if you live in the USA, anyways), if you want a 2-into-1 system. I ordered some from them, but with my wife's list of house-improvement projects I haven't gotten round to trying to fit them yet. They are a bit slow in their delivery, too.
http://www.cyclepages.com/ProductDetails.aspx?ProductID=4033017As the others have said, if you want a "classic" look, their are some stainless steel models that will likely last your lifetime, though they are pricier.
One of the more common stainless models I've seen are those from Epco (epcostainless.com)
I haven't measured them lately, but I do believe that the R45/R65 header diameter is 35mm, or maybe it is 38mm and the R80/R100 is 40mm? In any case, they ARE
smaller diameter (and since the cylinders are shorter, the exhaust ports are closer together) on the R45/65 than the R80/100.