I know this thread is old, but I wanted to add a note since apparently times are changing!
I bought original replica keys on ebay that match my one set of good Mustang keys I currently have and took them to Lowe's and of course they didn't have a cartridge for their magical electronic automatic key cutting machine, so they couldn't cut them.
Took them to Ace and they have a big sign that says they won't cut keys you bring in- only those purchased in store. When asked about that policy the employee told me it is due to liability in case they mess up your replacement key- they can't be responsible for damaging something they can't replace. I pressed them on it and asked them to just charge me for whatever blank they wanted and I'm willing to take the chance of them messing up one of my blanks but they said no. I needed to at least have an extra set so I had them cut two new ones (ign and trunk) with blanks they had, and the only one for the trunk that they said would work was one with a black plastic head on it (talk about not looking that part whatsoever), and low and behold that one won't even fit in the lock cylinder. WTF.
Do I need to contact a local locksmith now? Grrr... everything's harder with an old car, even just getting a basic key cut.
1967 Mustang C-code Coupe