The level 5 wizard changeling substitution level grants the supernatural ability morphic familiar that, in part, says
As a full-round action, the familiar can change its form to that of any creature that the wizard could normally have as a familiar (see page 52 of the Player’s Handbook). The wizard gains the new benefi t of the familiar while losing the previous benefit. If the changeling wizard does not have a familiar, this ability has no effect until he gains one. (Races of Eberron 123–4)
Assuming my DM is okay with it, my changeling wizard is considering this racial substitution level and the feat Improved Familiar (Dungeon Master's Guide 200) so as to expand greatly the options that he could normally have as a familiar.
(Note that a DM may not be okay with this, reading in that excerpt above the wizard as the wizard class rather than the changeling wizard or reading normally as only those options available in the Player's Handbook and excluding all others. In either case, that makes effort on this question wasted, so answers should assume a generous DM.)
Improved familiars that require an arcane caster level of up to 6 are okay—the Large viper snake (as per Dragon #341 89), for instance, or the Small earth elemental (DMG 200), the spider swarm (as per Dragon #329 98), or even the worg (as per Complete Warrior 100)—, but at arcane caster level 7 the myriad of mephits become available as improved familiars (as per Complete Scoundrel 78), for example, and mephits make available a host of unusual spell-like abilities, like the spell-like ability simulacrum of the mirror mephit (Expedition to the Demonweb Pits 208–9).
Just waiting until level 7 isn't an option as this is for a high-op E6 campaign. (I know that's counterintuitive, but don't judge.) Further, the DM has made it clear that a PC should be able to use his trick all day—only once or twice per day or being dependent upon consumables is likely insufficient.
With these binds in mind, is there a way for a changeling wizard 5 or 6 by himself and that has only 11,000 gp to his name to increase forevermore his arcane caster level to 7—ideally, on the cheap—so that he can use the supernatural ability morphic familiar to turn his toad into awesome?