I take NatureMade brand vitamin-D supplements.

That brand is USP certified which means their product is externally certified (through products purchased at stores) to contain the amount of supplement they say it has.

There are some other similar certification programs that you can look for when choosing supplements, but USP seems to be the most rigorous:

https://www.consumerreports.org/supplements/how-to-choose-su...

I registered a 5 ng/ml with a blood test. I wasn't feeling tired or seeing any negative effects at that time from it and most likely it had been low for a decade. However for obvious reasons I began immediately taking actions to move that number persistently into a healthy range.

I've been using a D3 supplement of 5000 IU from NatureWise, for several years. I take one per day depending on the time of year. It's inexpensive, ~$11-12 for a year supply, and it has worked very well. I take it daily throughout Fall & Winter, and then lighten up on it during Summer.

I take Jocko D3[1], it's $25 for a year supply. The dosage is on the high end though, it's 5000IU per dose whereas doctors recommend 4000IU per day. It's important to get bloodwork done once you take it, because if you aren't deficient on vitamin D, it can have adverse health effects if your levels get too high.

https://originmaine.com/nutrition/jocko-d3-gel-capsules-360c...

I take 5000 IU D3 a day, in a pill that combines 180 mcg of K2. K2 is important for calcium transport. My most recent tested level was 38 ng/ml. I'd like it to be above 40. I also make sure to get a lot of sun, because I think the measured benefits of D3 blood levels might be partly a proxy for sun exposure. A good resource for managing your serum D3 is https://www.grassrootshealth.net

Sam's Club has this really crazy-understated product (imo) that I like because it contains fish oil and 2000IU VitD in a fish gelatin capsule: https://www.samsclub.com/p/mm-fish-oil-dbl-d3-200ct-fish-gel... .

Thone 5,000 IU once per day (qd)