I like Eric's options, here!
Have you made the batch already? If so, a combo of pedio and brett cultures will give you sourness and funk in several months (8-12).
Sour fruit is a great option. Tart cherries or rhubarb are good options. With cherries, you can use puree (or puree frozen, thawed berries) and pitch lactobacillus before combining with the beer.
For clean acidity, I'm a fan of blending in acidic beer. Brew a small batch of wort with similar grain bill, but don't add hops. Use a sour mash/sour wort method or ferment with a healthy culture of lactobacillus. I like Wyeast 5332 (Lactobacillus Brevis).