I have the same issue, my piggies don't like pepper either.
Here's a list of high Vitamin C food, though these can all only be fed once or twice a week in moderation: Brocolli, kale, orange, brussel sprouts, papaya, kiwi. I recommend only feeding one fruit at a time due to their high sugar and acid. Parsley is good too, it can be fed 2-4 times per week.
In addition to veggies/fruit, it's a good idea to feed your piggies fresh forages for a C boost and for variety (chickweed, clover, dandelion, shepherd's purse, plantain, corn husks (inner husks only)/silk, cleavers/stickyweed, groundsel, fennel, yarrow, silverweed, vetch, raspberry leaves). Dried forages are good too but won't really give extra Vitamin C. Also, pesticide/fertiliser-free grass is perfect to be fed every day. Don't feed lawnmower clippings, either handpick the grass or place them on the lawn in a protected run. Slowly introduce into their diet, but after they are very used to it there isn't much need to moderate it.
This here is useful:
http://www.guineapigcages.com/forum...vy-Nutrition-Charts-amp-Poisonous-Plants-List