what digital cables for my dac?

Whatever you get, though, make sure it's a 1.5 meter length
1.5 meter is better than 0.5 ?can you please tell me why , thanks

Feel free to try shorter lengths of digital cable. While some cables may sound better in longer lengths, the general rules about all cables still apply to digital, shorter is always less damaging to the signal being passed in the cable. I'd suggest using the most revealing digital cable you can get, and letting your analog gear/cables tweak your sound. I try to never give up resolution in order to gain pleasant tonality, and any tweaking I do, I do as late in the signal chain as possible.
JMO.

Let me suggest you try the Audio Horizons digital interconnect. I use it. It sounds wonderful. It's in your price range. It's available for free trial period.

Duypham,

I know the use of a longer length seems counterintuitive to those of us who are used to "shorter is better". Shorter _is_ better, for analog cables.

Go to http://www.empiricalaudio.com. Scroll down to the link "Paper on S/PDIF cable length in PSOnline".

In a nutshell, this paper explains that signals are reflected internally in digital interconnects. Good DAC design can minimize these reflections but this is usually not done. In shorter cables, the reflected signal returns to the output very close to the rising edge of a wave, and confuses the DAC about the timing of the signal. The result is jitter. The longer length allows the reflection to return at a time when it is less likely to be confused with signal. The result is more coherent music.

(I apologize for any inaccuracies which may be found in the very brief summary above.)

You can also refer to Chris Ven Haus' Web site, where he specifies that 1.5 meters is optimum length for digital interconnects, and to UHF Magazine number 74. UHF tested different cable lengths and now do not recommend any length shorter than 1.5 meter.

UHF Magazine

I did my own tests and came to the same conclusion. I used 1-meter and 1.5-meter lengths of 75-ohm Apogee Wyde Eye (still a contender for best at its price point). We listened using my TEAC VRDS T1 and Apogee DA-1000E-20, and also a friend's Theta Universal and Kora Hermes. The results were unquestionably in favour of the longer length.

You could try a test yourself and see.

It seems that most of the analysis of digital cable length is based on theoretical levels of reflections in cables. These usually assume some horrible mismatch between transport and dac, or a digital cable improperly terminated. If we are talking about a properly designed and terminated cable, then other factors besides reflections come into play; such as bandwidth attenuation caused by inductance and capacitance values that increase with length.
If we're comparing apples to apples, quality cable in different lengths; then I suspect that much of the praise for longer lengths is due to personal sonic preferences.
I used to use the standard 1m lengths in my system, but then I discovered that my setup prefers .5m. It's more open, and less smeared and colored in the midrange and treble.
Maybe the best bet would be to buy some decent cable at various lengths, and see what your system 'likes' best? Then you could use that length in further listening tests with other models of cable.
Okay, so that's maybe not the most practical option, but what about this hobby is 'practical'? :)