I am also wanting to build a refractor. A 4" or 6" f/15. Istar Optical is currently selling 6" objectives in various f ratios for just short of 800 USD. I have noticed about a month or two ago someone was selling a lightly used 6" f/10 objective in the classified ads for a considerable discount. Edmund Optics has 102mm, 116mm, 128mm, 140mm Large Achromatic Lenses in stock for about 1200 USD.
There are many optical shops that will design and build a lens for you. There is LuminFlow, LLC located in Zeeland, Michigan. The company I work for has used them in the past to design and manufacture custom lenses for a product.
A short list from the Photonics buyers guide.
Align Optics, Inc
Apollo Optical Systems, Inc
Applied Optics, Inc
Global Precision Optics
Lumin Flow, LLC
Rainbow Research Optics, Inc
Rocky Mountain Instrument Company
These suppliers can design, manufacture and package a lens for you. And all of these are located in the USA.