The hub you have now isn't a flip flop?

Cheapest is probably to buy all new spokes and relace the wheel. Your problem isn't uncommon. The wheel wasn't built with enough tension initially. The spokes have fatigued enough that they're failing early. You're right - since you've broken two already, you can expect them to keep failing.

Formula makes pretty cheap, but good quality hubs. I think there might be cheaper ones out there, I can't remember the name