Stews, or, Spicy Stews, are up for grabs after the Evil Dave RFD part. Different colored spices give different boosts, ranging from -6 to +6, although +6 is extremely rare and should never be relied on. In this case, you'd like Brown Spices. My point being, they aren't buyable, and unless you have a Cat or a Wily cat, a pain to get. As for getting boosted levels for longer, I'd say the Berserker Curse is your best bet, although I'm not sure if they work on Stews.