The big one is Sword of the Arcane Order from Champions of Valor, which allows a Paladin to cast wizard spells from his Paladin spell slots. The tricky part is that its interaction with Battle Blessing isn't totally clear. For folks who run with the belief that D&D rules are written with absolute precision and consistent usages, SotAO does refer to the spells that a paladin can cast as "wizard spells," which may rule it out from cooperating with battle blessing's "paladin spells," but there's a case to be made for either side and pages and pages of bickering to go with it, so flagging it as a potential [amazing] add-on is probably enough.
The next easy way is an Initiate feat, but they tend to be a much smaller scale and come with a second benefit that's occasionally pretty good. Notable domains for a Paladin are Initiate of Baravar Cloakshadow from Champions of Valor (adds Camouflage, Invisibility, Displacement, Dimension Door and lets the paladin burn a turn attempt to reveal all nearby illusions) and Initiate of Helm from Player's Guide to Faerun (the big draw is Mace of Odo to make 10d6+10 force touch attacks that may or may not be power attack-friendly, but forceward's also nice, and the feat also gives the AoO-while-flatfooted half of combat reflexes).
Domains may also work, but they don't explicitly add their spells to a class's spell list, which runs into the same problem SotAO does, where you can get bogged down in what a "paladin spell" is for Battle Blessing's purposes (a spell on the paladin's class list? a spell cast from a paladin spell slot? a spell on the paladin's class list that's cast from a paladin spell slot? etc.), and there are plenty of ways to get domains.
You could also google the old "Ways to expand a spell list" handbook/thread from brilliantgameologists dot com (This forum understandably doesn't like new accounts linking things), but it doesn't give great parameters for what's available to a paladin and what's not, even before factoring Battle Blessing's potential weirdness.
