My brother supplied the materials.
When I was 13 years old I asked my 16 year old brother, who had just gotten back from a date, what he had for dinner. With a grin, he said "guess" and had me sniff his fingers. At the time, I think I guessed something like "french fries," making him laugh. This happened a few more times when he would return home from a date and tell me to sniff his fingers so I could guess what he had for dinner. Finally, he told me that the scent was the essence of his girlfriend's pussy, whom he had been fingering and later fucking. From that day on, I'd beg him to sniff his fingers so I could smell the aroma of cunt juices and throw a boner. I would always jerk off as soon as possible, trying to keep the memory of his girl-friend's twat in my head and imagining my brother finger-fucking her and shoving his dick deep in her cunt. I think all this secretly pleased my brother and gave him the sense that he was providing jerk-off material for his little brother. I didn't realize this at first until one day he brought home a pair of turquoise panties that were soaking wet. He threw them to me, telling me I could do whatever I wanted with them, so long as I returned them by the next night he would meet up with her. When he got them back he laughed, calling me a perv, and asked if the visible stains he saw were cum shots I had deposited. I admitted they were. He returned the panties to her, telling her they were his own cum stains. Over the following years, his "supply" of jerk off material for me (besides porn links and files) included pussy-shots of girls he was seeing, dirty panties, and even a couple of sex videos of him fucking them or them sucking his dick. Now I've started fucking girls and finally I'm able to trade with him too... occasionally slipping my fingers under his nose, asking him to guess what I had for dinner.