Devoted Performer from Complete Adventurer page 107 is a multiclass feat that stacks paladin and bard levels for the purposes of advancing Smite Evil. It also allows gaining more bard levels despite being lawful and you must remain lawful good to retain paladin abilities.
A bard can't be started as lawful, but a character's alignment can change over time. As per the bard's rules in the PHB, a bard that becomes lawful retains all the class abilities but can no longer advance as a bard.
As far as prestige classes go, it depends on what kinds of things you want to focus on.
Devoted Warsinger is not an official D&D class and cannot be found in any supplements. It is purely homebrew posted to a wiki site. For the sake of simplicity at filtering information ignore
all wiki sites except wikipedia's D&D pages since there is a lot of homebrew and unofficial stuff on them. For example,
this wikipedia page lists perhaps all of the prestige classes in official rule books.
For what it's worth, if Devoted Warsinger were an official PrC I would not recommend using it because the Code of Conduct class feature means you cannot retreat from a battle lest you lose all your prestige class abilities until you get an Atonement spell cast on you. Pure and total BS.
The type of character you're thinking of is probably best done as a bard and crusader. The crusader can be found in Tome of Battle, but don't expect your DM to allow it because many think it's too powerful (or too "wuxia") despite the fact that spellcasters can easily do much worse such as rewrite reality.