As Rds says, South America is (in my opinion) the best place to get players. They're typically cheaper than European alternatives so have lower risks. Careful not to get sacked and leave all the 4/5 star potential players with Wolves though. I did this with Lincoln who I bought for Villa at a measly price of £4,000,000, got sacked after a few months (before he'd even joined) and decided I'd try to get him when I got the Arsenal job...£30,000,000.
I'm sure @Daniel and @tarjei have better advice based on their ridiculous saves.