Mostly used is pyronin Y. Look at http://onlinelibrary.wiley.com/doi/10.1002/cyto.990080206/pdf or http://www.flowcyt.rutgers.edu/main%20pages/Protocols/DNA%20and%20RNA%20Quantitation%20using%20Pyronin%20Y%20and%20Hoechst%2033342.doc

The Molecular Probes handbook continues to be a useful reference for fluorophore information.

Have a look at the following link (I used tinyurl converter since it's a long string otherwise).

http://tinyurl.com/nzb25ex

SYTO RNASelect Green-Fluorescent Cell Stain should suit your need, and there are other choices also described on that page.

Thanks!

looks good, I try it