It has been a while since I bought a decal set. But I recall the 'outboarddecalset.com' site that you show a picture of and there was also 'discontinueddecals.com' and 'outboarddecals.com'
You can also find them on eBay.
As I recall, 'cheapest' was not what you would call any of them . . . I think I paid $50 for a set of Mercury 7.5 HP decals. I also got a set for my Johnson 4.5HP . . . just don't remember the exact source or cost.
If you are doing an engine make-over, the decals sets tend to be one of the more pricey items.