Can we still by freeze plugs locally (if so what size are they) or do we have to order from a Model T parts vendor ?
Yes of course. Show the guy at the part store a nickel. The freeze plug that you want is exactly the size of a nickel, if the nickel was cupped.
This is why many of us simply use nickels. A freeze plug from the part store usually costs fifty cents.
Auto parts store
.8125 dia plug, soft steel.
IMO, use brass, the soft steel will rust out in 3 years, mine did. I always order the brass ones from Lang's or Snyder's.
Or you can use real Buffalo nickels, they fit and look better
I second Dan's advice to use brass. I installed steel plugs in my engine and they lasted just over one year before starting to leak. I replaced them with brass and no troubles for over 10 years.
Real nickels - those made before about 1980 or so - also don't rust. I started installing them with JB Weld, it gives great results compared to any of the sealants.
Here is another thread on the subject: