I had low iron stores and low b12 a couple of years ago. (Funnily enough my folate levels very quite high though - which can in itself mask a b12 deficiency)
I was getting really tired, all the time, to the poin of falling asleep, not being able to drive etc.

My Dr did some test to check for the reason my B12 was low, because there are a coupld of things that can cause it - mine was just dietary though, which is easy enough to fix up (well, harder for me as I'm
vegetarian) - I did take a supplement, from the chemist, it was a dissolvable pink tablet, can't remember what it was called though.

For the iron, I initially took a supplement 'ferrograd-C' which has vitamin added to assist in absorbing the iron. Also tried to focus on eating better, which did the trick cos I ended up with more than enough iron and had to stop the supplements.

DH had low B12 too, his wasn't dietary, his body doesn't store B12 properly so he's supposed to have injections every 2-3 months.