Unless your speaking of one of the major manufacturing companies pistols there are no "drop in" barrels made for pistols.
I'm pretty sure Lyman, Pedersoli and Traditions will sell a replacement barrel but I may be wrong.
As for unfitted pistol barrels I've used one made by Rayl. Excellent workmanship and quality but, as I say, these barrels are not drop in parts.
They must be breeched and fitted to each stock.