I've found that it doesn't matter how many flags they have. People with green flags might give just as much as anyone with a platinum flag.
Use the Secret Base function of your map to track down where all the hidden bases are. You can use it to fly directly to that route. Then take their flags, and systematically invite them all to your base 5 at a time. See if they give any flags, and then get rid of them again. Rinse & repeat until you got all Secret Bases. Delete them all before you update the BuzzNav again (you can do this from the PC in your own base as long as you've visited them before). Bases won't give as many flags, or none at all, if you've invited them before, so make sure you get new ones every time. You should be able to re-update the BuzzNav about every 8 hours, which means you could do this twice a day if you have that kind of time.
You should be able to get roughly 30 flags per day at least. Depending on how lucky you are, you could get over 100 per day. I've sometimes had a Secret Pal giving me roughly 30 flags, and he wasn't even the only one that day. :P