I just talked to my aunt last night. There's quite a bit in season. She brought up Mangoes, Caimito, Watermelons(you should try these heirloom variety), and if you like Jicama(singkamas), those are in season. Sweetsop, Santol, Durian, Mabolo, Manila Tamarind(camachile), Purple Mombin, Wax Apple, Jambolan(duhat), Mangosteen, Soursop(guyabano, Jackfruit(langka), and Marang may be available if you look.
If you're in Manila, there's open air markets, but what I like when I can't find rare fruit in the open air market, the supermarket. If you go to SM supermarket at the mall, the grocery usually stocks fruit from specialty farms. I've bought Mangosteen, Marang, Wax Apple, Soursop, and Lanzones at the SM Mall of Asia.