Yeah, I dont know about the same threads and size, not to mention that tank thickness. All of my c02 tanks seem pretty thick, where propane tanks seem thin. I dont think you will find any places that let you fill your own c02.
Primaries: Mojave Red (AG)
Kegs: Hibiscus Saison (AG), Orange Kolsch (AG) , Cocunut Porter
Future Brew: Wee Heavy