If you want genuine, try your local Yamaha Dealer, but take a seat before he tells you the price.
E-Bay is the next option, either for the balljoints, or aftermarket A-Arms.
The balljoints used in the aftermarket a-arms are far more readily available than the stockers.
Also putting aftermarket a-arms on gives you the option to go wider & longer at the same time.
The 660 is too narrow, & too tall stock.
Look for threads with regard to flipping your rear link to lower the rear.
I wrote this one on RF
Get 2" spacers for the rear hubs (or G-Force extended hubs)
I also put ASR A-Arms on (+3+1) for my build & they seem to be strong & well made.
I wouldn't buy anything cheaper, but you can pay an awful lot more for suspension than them.
P