TDS and EC are, essentially, different units of the same measurement. TDS is total dissolved salts. EC is electro conductivity (i believe) which is the amount of current that a liquid can transfer. The conductivity of a liquid is directly related to the amount of dissolved salts in the solution.
So basically you should be able to work with what you've got.
do a google and I'm sure you'll be able to find a conversion for them.

Thanks for the information Grizzman! I did not know they were similar.

I did find this calculator:

Here is a link that might be useful: PPM to EC conversion