You should able to get the reagent from Sigma-Aldrich (or their distributor). Does running the reaction in reverse help? (I'm not sure how reversible it is). For example, if you cannot get NADH, could you run the reaction in the NAD to NADH direction instead? Of course, if the cofactor is essential and you cannot get either form, I don't see how you can proceed.

Thanks @Nick, I have already considered running the reaction in the reverse order. BUT getting the product (NAD+) is as difficult as getting the reactant (NADH) in this area. Meanwhile, I have contacted an agent from Sigma-Aldrich to see the possibility of getting it across.

Baker's yeast is a good source. You can buy from super market, soak in 10% glucose solution for 3-4 h and then grind with sand in 0.05 M Tris-HCl buffer to get cell free extract containing NADH. Centrifuge to get a clear solution. You can use this. If you have a membrane filtration equipment you an also remove most of the proteins if necessary. I am sure there are experimental procedures available on internet for isolation of NADH from baker's yeast.

This is a great idea @Nitin, I will surely try this out! Much appreciated 🙏