This is how I got my 5*s. Hope it helps.
First, I got army of near maxed good 4's, hogged TEs as much as I can (many people sold 4* for luna back then), and converted Potions for TEs (you probably don't want to do that now, since Potion is almost equal to TEs). I was trying to sell max 4, but the second coliseum event kicked in, and the event gave tons of TEs and Potions (3000 pts for 3 TEs and 30 wins for 3 Potions?). So, after a month and half, I was ready to buy 5*
1st 5 was Meg for 50TEs. It was back then when max Meg was worth 80-100 TE. She still is pretty decent for a price, but her trading value is pretty low even now.
2nd 5 was Sheryl. Took 150 TEs + two 4 max. I might have overpaid a bit, but loved her from the beginning to the end. I bought her for trading value, stats and look, but her trading value wasn't quite as high as I expected, because there are too many Sheryl.
3rd 5 was Mihiro for 55TEs. I got her, because I needed a strong magic user and didn't have much to spare. She was cheap, have great stats, and her 25 deploy cost beats 30 deploy cost of Yuki. Still think it's a decent purchase.
4th 5 was Burnhard. Sometimes, top players trade their 5s for max 4s, and I was able to get him for four 4 max. Could have gotten Sheryl for five 4 max too, but just missed out. Visit top players once in a while, and you might be able to find bargains that you can't find in here.
5th 5 was Sonia. I was able to convert her in 2nd try.
6th 5 was Alester. Traded max Sheryl for her. Tears my heart to let Sheryl go, but Alester's trade value is higher than Sheryl, so I couldn't pass this deal...
So yeah. Even without buying the mega box, you can still get 5's in multiple ways...