Don't forget garage sales and thrift stores!
As for formal nights...there is NO requirement to dress formal in the dining room! Any dress, skirt or slacks will do. It's your vacation and you should just enjoy everything, including the dining room! I wear the same moderately "formal" cocktail dress on both of the formal nights in the dining room - bought it on sale for next to nothing. I won't be seeing any of the other passengers again and don't care what they may think of my attire. I do what I can afford to do and have a great time with what I've got! Going on my 9th cruise in 3 weeks!
In fact, I find that most people are so focused on themselves...that they couldn't tell you what you were wearing an hour after having seen you! :o)