I have experience using feed with terramycin. I use it only when I really need it, and it is a good idea to keep some on hand a head of time rather than waiting for it if you need it right now.
I would suggest ordering feed with it already mixed in. Many of the major feed suppliers have some on hand, although it may be smaller in size than you are normally using, and it will probably be sinking. I just ordered some feed with Romet in it which is more of a broader spectrum antiboitic than terramycin. I will have it on hand if I have problems with my feed trained bass fingerlings and brooks when I get a sudden warm up. The brook trout are especially sensitive to sudden temperature changes and can develop aeromonous problems as a result. (The aeromonous bacteria is always present in the pond)
Remember if you feed medication to your fish you should not eat them for 21 days. Fortunately my fish are not raised for consumption.
I'm getting Silver Cup feed now from Nelson and Sons in Utah.
http://www.users.qwest.net/~silvercup/trout.htm