Your doc can just order the DTaP from whoever supplies their vaccines.
My dh is a dispensing GP so has a drug wholesaler who he gets it through - as he's only ever worked in a dispensing practice I doubt he'd know about how a 'normal' practice gets it.
He offers DTaP to all parents, some accept it, some want the DTwP because of the link with Hib being less effective. What he does, which most other GPs don't do, is offers a CHOICE to parents on this one. Our 2 older children had DTwP, as we didn't know anything about it, but as my family (on both sides) has a very strong history of Rheumatoid Arthritis (which is an auto-immune condition) and I've already had a +ve Rheumatoid factor blood test though it doesn't affect me at present we decided that as we now knew more about it dd2 would have DTaP. After we did the research (having found out about it on MN to start with!!) dh decided this is what should be offered to parents in his practice as well.