If you want to go for a Multifix type toolpost I can recommend Create Tools in China – I have ordered twice from them in the past, and several guys on the mig welding forum have done likewise. Quality is excellent.
I think Rotagrip also sell them, and though they are usually more expensive than direct from China it is worth checking as they sometimes are on special offer.
I would think the Aa size would be the one to choose for a Myford, but you might be able to use the A size – check the sizes carefully before ordering, there are drawings on both Create and Rotagrip websites I think.