Non excisable unit can supply excisable goods without excise duty

Do you mean you want to buy the same material from 2 different vendors: (1) who charges ED (2) does not charge ED? If yes, then you can buy from both or any one whatsoever

You can buy from anyone no problem. 

You can buy from any one only you will not get set off for non excisable goods

nobody can sell goods (manufactured in India) without excise duty-either it is manufacturer or dealer. manufacturer imposes excise duty & excise registered dealer just pass it if they are selling to other manufacturing company. seems like 1st company you're talking about is under SEZ. in this case you can choose any of these. if you are excise registered just check whether you'll get excise benifits if you're buying with excise.