http://en.wikipedia.org/wiki/Citric_acid#Occurrence
47 grams per liter of lemon/lime juice. That converts to ~25 g oxaloacetate per liter. Oranges apparently have less citric acid, to the tune of perhaps 500mg oxaloacetate equivalent per liter of juice.
Food-grade citric acid (also sold under the name "sour salt", usually shelved with spices) is FDA-classified as GRAS. Looking at Amazon, the Spicy World Citric Acid in the 5-pound bag is $19.23 (free shipping for me, since I have Amazon Prime).
At the ~2g of citric acid metabolizing into ~1g of oxaloacetate you suggest, that translates to a price of $0.05 per three grams of oxaloacetate, or three orders of magnitude cheaper than buying a bottle of 30 100-mg capsules for $49.
For those who are interested in this topic, I'm just wondering what longevity research today looks most promising to you and why. Whether that's SENS, cryonics, nanotech, brain uploading, etc is fine with me. Any links to actual research papers would also be greatly appreciated. I'm very interested in longevity, and am curious to see if anyone else would like to offer some thoughts on the current state of the art.