Where can I get something better than a Bronze Edge for Frog?

I've finally managed to rescue Frog from his sulking with a really expensive gift. However, he can't use that gift. For that matter, I'm having trouble finding something he can use. I travelled all the way to 65,000,000 BC to get him better armour and magic powers in the process, but he won't touch that Flint Edge. He doesn't care about 1,000 AD Red Katanas, either. As for AD 2,300... it's a looong way to the item seller and I'd rather not travel it for naught.

Frog's now locked into my party, but he's underleveled (level 19 vs others' level 21, but that's what grinding is for) and underequipped (a very worrying 43 damage to my 70 or Robot's 74.) Okay, I know he can wield an Iron Edge, but I moved it to me and then sold it ASAP and the seller has updated his stock after I... lent him a hand in the castle. Whooops.

What can I do?

You don't get the Masamune as a weapon until after a cutscene. When you get Frog back into your party, head to the North East of the forest you got him from. There should be a mountain (not the one you got the broken Masamune from). Go to the base of that and enter it. There, you will get the cutscene. Afterwards, Frog automatically equips it. It's important to also equip him with the Hero Medal, as that increases the Crit rate of the Masamune. Also, you will want to use the Masamune during the fight with Magus, since it's a powerful weapon against him.