NMN is not approved yet but you can buy them online from reputed sellers. Check their 3rd party testing. Dont buy from Amazon/Ebay, some of independent tests came up with 0% NMN (so what are they?)
Vitamin B3 also increases NMN level and is a precursor to NMN.
It's probably better to just take nicacin (B3) as its safer until NMN is proven to be safe for human in longer doses. NMN is too large molecule to enter cells so anyway its broken down before it can be absorbed.
B3 is a precursor to NAD+ but you also need some amino acids to make it.
Taking NMN directly also had some health issues that were discovered recently. More importantly it can speed up cancer if you are predisposed to it or if you already have undiagnosed cancer.
as far as I can remember, one of the treatments of cancer is removing nmn from body.
You can buy niagen, which has nicotinamide riboside (NR), which is basically the same thing - the goal is to increase NAD+ in the mitochondria. NR is an immediate precursor to NMN which is NR with an added phosphate group, which makes NMN itself unlikely to enter any cell directly.
Vitamin B3 also increases NMN level and is a precursor to NMN. It's probably better to just take nicacin (B3) as its safer until NMN is proven to be safe for human in longer doses. NMN is too large molecule to enter cells so anyway its broken down before it can be absorbed.