Like domo said, what are you looking for?
Do you like the MT look or the AT or the standard HT or are you looking for a race look like the proxes?
Before you just get adapters and try and make the 19s fit, you should ensure the 19s are rated to hold the loads the Frontier will see. If they don't meet OEM specs, then they are going to be a danger. I can only imagine you having something in the bed, take a turn and snap..... you get the picture.
Plus, using adapters will push the wheel out and will cause rubbing issues when turning.
Lastly, 19s are not common and will be hard to find. Might as well just buy some 18s or 20s in the correct pattern w/ tires and sell your 19s to make some cash. This is going to be the smart move.
P.S. 2nd gen Frontiers are hub centric so adapters are not cheap and if you go cheap, you really run the risk of having vibrations and other issues.