I may as well go over a few other options to expand your spellcasting potential a bit more thoroughly. First, let's go over use of the
Paladin's Divine Bond with the mount option (or obtaining an animal companion as a domain power of the
Animal or
Scalykind domains through
Sacred Servant). If you take the mount bound for an animal companion (preferably you would want to use
Monstrous Mount to have a
Griffon as your companion as opposed to a horse - the mastery feat is not needed), you can archetype it with one of two following archetypes:
Feytouched and
Totem Guide. Both of them allow you to sacrifice spell slots to spontaneously cast spells the companion provides you, but the Feytouched Companion only lets you cast them 1/day. Their spells do go above 4th level, but the higher levels don't matter for Paladins.
Feytouched Companion:Easy enough to say Deep Slumber is worthless. You'll be level 10 before you can use it. Another problem is that you give up share spells, so you cannot give your mount
Divine Favor or similar should you pick this archetype, unless you take the
Improved Share Spells feat or
Improved Spell Sharing teamwork feat (which will require either a casting of
Shared Training, a
Pack Tactics feat, a
Sisterhood Style feat, or a
Ring of Tactical Precision, which will probably require either a
Hand of Glory necklace or an
Extra Item Slot feat on your animal companion). Although Faerie Fire and Glitterdust are somewhat redundant with each other, they still have individual uses (Faerie Fire is a lower slot and also negates effects like Blur, Displacement, Invisibility, and any concealment from darkness, making it much easier to hit people, whereas Glitterdust is also great for blinding, especially with a lesser metamagic rod of persistent spell) and Major Image can come in handy in a pinch, although a fourth level slot is an expensive price to pay for it.
Totem Guide Companion:With this list you'll probably want a
Magical Knack trait and perhaps a
Messenger of Fate feat or
Orange Prism Ioun Stone. Normally the restriction against changing your animal companion is stifling, but when you're using a mount bond as a Paladin your options are very limited and a Griffon companion is pretty much the one companion to have.
Another feat I haven't mentioned yet that can expand your spell list is
Sahir-Afiyun, which gives you access to a lot of extra spells. It's a bit sketchy though, since it involves drug usage, but Pathfinder does not in fact classify drugs as poisons (despite the similar mechanics), so I guess it isn't strictly against the code of conduct. However, you would have to follow consent carefully and causing drug addiction may be cause for falling, and it's easy for a GM to thematically read this as using drugs on enemies, which is probably grounds for falling, which would limit you to only friendly spells (like Augury, Divination, and buffs). The Paladin is immune to drug addiction though, since that is technically a disease and Paladins are immune to those (as is anyone wearing a
Periapt of Health).
You can also try taking the
Dreamed Secrets feat, but worshiping one of those gods is extremely dubious for a Paladin.
It should be possible, however, to expand the Paladin's spell list with a number of illusions at level 16 simply by taking
Deific Obedience (
Grandmother Spider). Note that neither
Deific Obedience nor the
Exalted prestige class require you to be within 1 alignment step of your patron deity. (Pathfinder Society might be different about this, as it prefers to insist that Paladins must worship deities, unlike normal PF.) Normally Exalted performs best when paired with Sacred Servant (for the double domains), but as Grandmother Spider is not within 1 step of LG, you cannot worship her as a Sacred Servant. The level 20 SLAs are not bad for a Paladin either, as your charisma modifier actually renders them useful and they give you a lot more options.
Next, there's the subject of the
Veiled Illusionist prestige class. As it turns out, Paladins only get two illusion spells on their spell list, both first level (
Inner Focus and
Oath of Anonymity), so in order to qualify for Veiled Illusionist the Paladin needs the ability to cast an illusion spell of 2nd level or higher. Feytouched Companion will qualify at Paladin level 13, but other methods will include
Sacred Servant Paladin (provided you use a suitable domain, like Trickery),
Hunting Paladin,
Wilderness Warden Paladin,
Unsanctioned Knowledge, the aforementioned worship of Grandmother Spider (which is a rather suitable deity for the Veiled Illusionist PrC that isn't Sivanah), or some other trick.