WATER WATER WATER and then more WATER
What kind of workout did you try? You need to get back into working out GRADUALLY if it's affecting your milk supply so much. (I had the same issue)
If just walking is fine then maybe try walking more 2x a day instead of just one. Additionally instead of say "working out" can you find some physical activity you enjoy and do that 3 x a week? Like dancing, playing catch, throwing frisbee, swimming for fun (not laps), etc. The idea is to get yourself moving more. Go for a walk and wear the baby. If you make an effort to start moving and just doing physical stuff your energy will improve but you won't be pushing your body hard so your milk should be fine.
Also, I found that if you do something like a Curves workout (I know some places it is all old people in there, but seriously Olympic athletes use the stuff too and it works) like 3 times a week you'll begin your body getting ready for more intense things. Curves has machines that a frail 90 year old woman can use and the Olympic athletes train on the same machines. The more effort you use pushing the machines the harder they work against you so the more of workout you can get. If you don't push hard they move easily. You could start going and just get the routine down (don't raise your heart rate or anything) and take it easy. You'll get the feel for the machines and the range of motion will increase your flexibility and even just the movements can speed up your metabolism. Most places offer a 2 week free membership so you can check it out.
I love it even though I'm definitely the baby there. I gained 95 lbs with my my 2nd child and Curves has helped me make it all go away.
Also you could rent/buy/check out from library LOW IMPACT videos ie yoga/pilates. The goal here is to just go through the routines at your own pace and just do what you can (don't try to stand on your head or worry if you're not as flexible). Low impact stuff can still help your health/weight without effecting your supply.
Water is the most important thing to put into your body!
Other than that just try to eat well and be HAPPY! Your happiness has more of an impact on your health and baby than anything else!