Advice on Vitamin D3 options?

I currently take 50,000 IU of prescription Vitamin D twice a week and my levels are still just slightly above normal.

My endocrinologist is lowering the dose and increasing the frequency, to 5,000 IU / day however this dose is not available by prescription; OTC is my only option.

I have sworn away all vitamins and supplements - generally speaking they are a multi-billion dollar unregulated pharmacy - and am generally ignorant of the quality of vitamins and supplements at Costco.

Are they reliable? Any indication of testing?

Thanks all!