You need a linear lnbf, and possibly a bigger dish (you didn't say which DTV you have-some of them can be fabricated on to work with some stronger fta signals).
Best bet would be a new 1m dish made for free to air, most pay-tv dishes are too small to work reliably and they usually have only circular lnbf for the stronger, pay-tv signals. Linear ku band, which is where most of the free to air comes from, is weaker and takes a bigger dish+linear lnbf to receive. Or, you can look around the area, check craigslist, etc for a no-longer-used primestar dish, there's still some of those to be had, usually just for taking out of somebody's yard. They are very nice fta dishes and the lnbf most always is ok and working.