This is an old answer and it’s no longer the best solution, please see the accepted answer instead.
-(UIFont *)boldFont{
//First get the name of the font (unnecessary, but used for clarity)
NSString *fontName = self.fontName;
//Some fonts having -Regular on their names. so we have to remove that before append -Bold / -BoldMT
fontName = [[fontName componentsSeparatedByString:@"-"] firstObject];
//Then append "-Bold" to it.
NSString *boldFontName = [fontName stringByAppendingString:@"-Bold"];
//Then see if it returns a valid font
UIFont *boldFont = [UIFont fontWithName:boldFontName size:self.pointSize];
//If it's valid, return it
if(boldFont) return boldFont;
//Seems like in some cases, you have to append "-BoldMT"
boldFontName = [fontName stringByAppendingString:@"-BoldMT"];
boldFont = [UIFont fontWithName:boldFontName size:self.pointSize];
//Here you can check if it was successful, if it wasn't, then you can throw an exception or something.
return boldFont;
}