Around this time last year, the 'gap' niche really kicked off. People started taking the realm of fitness pretty damn seriously and Instagram chicks were popping up left, right and center that knew they could cash in on their small thighs and ability to filter a semi-decent picture. Anyway, /r/DatGap is a subreddit with a pure focus on that type of material and as far as I can tell, is probably one of the few places left on the net that still actively shares this type of material.
There are 75,000 readers and new submissions are made about once every three or so hours. The rules are pretty standard with submissions needing to be via Imgur, no voyeur content submissions and for-profit content being banned. /r/DatGap typically sees a lot of amateur content, but professional girls also get a mention from time to time. User submitted gap porn images are rare, but they do come up from time to time.