Well, in Urban Arcana, there are rules for Incantations, basically high level spells that requires more time and effort then the lower level ones. Also, you could always create a prestige class that extended the spells up to ninth (a 5-level one, which is the standard, would get you up to 8th level spells if you use the standard progression, and probably even higher if you you a stunted progression).
Also, you can limit the high-level spells to dragons and their kin. Or, you can take a page from Eberron, and have a high saturation of magic, but overall a lower proficiency, with high technology or the aforementioned Incantations replacing the 6th level spells and above.