Start collecting cave keys from events like myoboku etc. Also consistently buy 20-30 keys daily from the new store. You will have 800 or so in a month and you can buy the remaining with coupons. Then use all on cave keys rebate. That's the only way for f2p to get gold orbs at an acceptable rate otherwise all the gold orb rewards are either rebates or top 3 in heavy paid events. Of course you c*ways get lucky and get 1 for free in a draw.

PS: I am also in the no gold orb club. Just recently started ac*ulating cave keys.