I suggest any of the gas distributors, such as Suburban or Amerigas dealers. Unless the laws have changed, you cannot change the valves on the horizontal tanks, due to their design. The old horizontal tanks are exempt from having the OPD valves installed.
Also as I understand, all new horizontal tanks are required to have the OPD valves.
Wayne
EDIT: The following is from the Overfilling Prevention Devices (OPD) For Consumers. It is from The National Propane Gas Association 2002.
"Q: I have a horizontal cylinder on my recreational vehicle. Are OPD's available for that kind of service?
A: The 2001 edition of NFPA 58 (the LP-Gas Code) recognizes that horizontally oriented cylinders that were manufactured prior to October 1, 1998, are unable to be retrofitted with the OPD's. As a result of this fact, the Code now exempts these cylinders from having to be retrofit with OPD valves. Any such cylinder must have a label affixed to it to inform the user and the refiller that an OPD valve is not installed. (Note: This provision was not contained in the 1998 edition of NFPA 58.)"