what digital cables for my dac?

Although the starting point using a "cheap Sony player" seems to be your weakest point, the upgrading of digital IC will let you know when you have heard all there is to hear from your transport, in the digital domain.
I can recommend using matching Mapleshade digital IC's (on either side of your Monarchy DIP), inexpensively excellent without being too revealing of transport inadequacies. Also, can point you toward a subsequent upgrade to match a more capable transport in your future: Auricle Audio Design Encore Signature digital IC.
Audiogon member Joemazzaglia sells these custom crafted digital IC's which truly excel at revealing everything your digital components can do, which is much more than you expect, if accustomed to most other IC choices. Priced too low to elicit budgetary shock and awe, which is a good thing.

I'll second the VH Audio Pulsar.

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.