Most places here on Phuket won't rent you tanks, but on the smaller islands you might get to know a shop by talking with the staff and explaining your background, and then maybe they'd let you take their tanks. If you're going to be in the area for 6 months, you could just buy a couple of tanks and get fills pretty easily then sell them. We don't do much shore diving in Thailand, though it's possible to get a longtail boat if you're staying at a place with dive sites nearby enough to drive out to them in a small boat.
I can't say much about Indonesia and the customary procedures there, but there is an SB member who has been active on the board recently and who's based in Indonesia at a place where there's plenty of close-in diving--instructorinasia is [-]his[/-] her username. I'll let [-]him[/-] her know about this thread and maybe [-]he[/-] she can offer some insight.