One way: Spontaneous Divinations (CC) + Drake Helms (Explorer's Handbook) can get you any arcane spell. Any scroll - whether typically arcane or divine - bought from a Warlock, Geomancer or Southern Magician can be arcane. Those scrolls can be used to calibrate the Drake Helm. This isn't practical in a campaign, but it can work in an arena setting.
Or RAW Polymorph, according to K's interpretation. But that's even more abusive. At least have the decency to use Assume Supernatural Ability or Shapechange.
In a campaign environment, Wizards can use indirect means like Summoning, Planar Binding and Domination to generate spell effects. (If there's a spell, there's an Outsider or Elemental that can cast it.) If you want to be the one casting the spell, the BoED has rules for Binding Outsiders.
The most practical way is to independantly research a modified version of the effect you want.
There's also UMD, but that's not really a Wizard's strength. Without PrC's anyway.