Does it have to be single viscosity; i.e., SAE 140, or can you use the multiple viscosity oils? I know AutoZone has the Coastal 85W140 and I'll bet most other auto parts stores do, too.
If you decide you need single viscosity, try a petroleum specialist. Every city has them. I used to have to get aviation oil from them and they stock various oils. They are probably more knowledgeable than the average AutoZone type employee about oils as well.
Yes straight 140 weight is available but usually only from people like me who sell oil. You will probably have to contact a local oil distributor to get a straight weight these day's. The best thing to do is go with a multi-viscosity oil. My suggestion would be to use a 85W/140 if you plan on using conventional oil. You will get all the benifit's of easier operation at cold temps while maintaining the film strength of a 140 weight at operating temp.