Of course there's no substitute for a real, well-tuned tanpura that you can lay your ear against. However, if you are determined to simulate its sound, the following free mp3s can serve as substitutes.  I hope these are useful to you.

If you have the means, iTabla Pro is a reliable iOS tanpura/tabla simulator that can produce realistic tanpura sounds in any sruti.

The pitches listed below are approximate--they are not tuned to any standard pitch. 

[right-click or ctrl-click and select "save as..." to download]