US: https://nutricost.com/collections/protein/products/nutricost...
AU: https://www.bulknutrients.com.au/products/whey-protein-isola...
edit - I should add, you can get a bit of an idea of the quality of the protein powder by just looking at the percentage of protein in it. Nutricost is 85% and Bulk Nutrients is 87%. Doesn't leave much room to shove other ingredients in with protein levels that high.
You can also go buy 20kg bags of WPI from NZ (which is where I think most of these retail products get their WPI from) https://www.nzmp.com/global/en/products/ingredients/types/pr...
[1] Quest brand: https://www.questnutrition.com/collections/protein-powders/p...
[2] essentially powdered egg whites are easy to find, for example: https://www.amazon.com/Its-Just-Protein-Non-GMO-Unflavored/d...
If purity is particularly important to you I'd recommend "isolate" over "concentrate", it will have even lower amounts of lactose, which can benefit people who are severely lactose-intolerant.
Pretty good stuff :)
https://canadianprotein.com/products/bulk-grass-fed-new-zeal...
They ship to the US for free, too.