In pretty much every place I've looked, bonus slots for sorcerers is extremely rare except in some very specific wildling themed items.
Either on faction gear/loot or on ambient drops it is very rare. Compared to wizards I think it should be a lot more common for compensation against the many advantages a wizard gets over a sorcerer in spell versatility, spell level access and so on.
Please consider doing a look through the gear available in various places (not just drops) and make bonus slots for sorcerers more common.
As a side note....it is also all pretty much "tribal/savage" themed sorcerer items, which not every sorcerer is.
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[TD]2 - Wizard known spells / day - Level 8[/TD]
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[TD]3 - Sorcerer known spells / day - Level 8[/TD]
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I disagree with using Wizards as the benchmark. I do however think that Sorcerers could use more gear / spell slot stuff that is not of the tribal / wildling aspect.
There should be some minor bonus for choosing to be a wild based sorcerer, its uncommon enough a theme as is.
Sorcerers get access to the same spell levels about 1 level after wizards usually, Sorcerers getting access on even levels and wizards on odd levels a level before (generally), you just happened to look at a level where they are both on the same spell level.
The point more being that for the loss of versatility in spells they should probably get access to more spells/day through bonus spell slot gear that isn't so exclusively tied to certain themes, or to put in example, most decent sorcerer gear is either tribal or necromantic and if you are neither you don't have much to choose from for drops/factional gear.