HEYU15
These guys have become quite a bit dearer since they were acquired by Getty Images a couple of years ago, but they still have one of the better-quality 'affordable' collections in my opinion.
I've bought credits from them a few times over the last few years and thought I would share the coupon code they sent me in case anyone else can benefit too.
You can also use AVERYWEB5 to get 20% off 50+ credits. I just bought some a minute ago… US$60 for 50 credits (down from US$75).