Roses are known by all I hope. Hips are the fruit ball formed at the end of the stem below the flower/sepals. Hips on bushes may not be widely available this time of year but I know a few places where they've gone wild and last year's hips may be still hanging on the bushes. Not sure of the C content in an overwintered hip but the flavor would have the tart flavor if C is present. Plus, pretty sure teas with rose hips are still plentiful.
Also look for teas with hibiscus as a top ingredient. Very high in C as well. Many peppers also have a high vit C content. A quick online search would give info on the content in specific peppers. I overwinter pepper trees and harvest year round.
check out Wneighbor post upthread # 73 iirc