To begin, I've had a Firestone model A keg sitting around for years without posts. All of my other kegs are set up for pin locks. My problem is that the posts/valves cost about $25 each from my LHBS. And then, they only have ball lock, so I would have to buy the fittings for my lines as well. So, we're talking $50+. This seemed a little outrageous...
Well, I've found a much cheaper way around this issue. The threads on my posts are 9/16"-18 and the threads for my hoses are 1/4" flare. Unfortunately, I couldn't find such a specialized fitting at Lowe's. I did, however, find a 9/16"-18 Tap.
I tapped the inside of a 5/8" brass flare fitting to the right threads and bought the various fittings to convert to 1/4". I didn't even need to drill it out first, the hole was already 33/64". I've (hopefully) included pictures below.
The only obvious problem with this is that once the keg is hooked up, the only way to disconnect it is to relieve the pressure and use 1/4" flare pipe caps to close it off. Kind of a PITA, but this was much cheaper and worked in a pinch.
5/8" Flare X 1/2" MIP
1/2" FIP X 3/8" FIP
3/8" MIP X 1/4" Flare