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]