You should be able to recoup some/most of the cost of a new RD by selling the 10-speed you have.

And to be clear, yes, you can use a 9-speed MTB RD for your 10-speed road shifter. I know it's confusing, but basically all of Shimano's indexed rear shifters move the RD the same distance per amount of pull from the shifter (with old Dura-Ace and new Dyna Sys the exceptions). Therefore, it's meaningless to even name a speed fort e RD because it'll just go where the shifter tells it to.

If you want to keep the Shadow look, here are the "9-speed" models to look for:
- Deore: RD-M592
- SLX: RD-M662
- XT: RD-M772
- XTR: RD-M972

Also in case you haven't considered this, before buying a new RD, know that the above come in two cage lengths: GS (short/mid-length) and SGS (long).
- Triple chainring crank: nearly always needs SGS
- Double: usually can handle GS, might need SGS
- Single: can always take GS (SGS fine too)

The GS and SGS versions will have a different rated "wrap capacity," its ability to handle the full range of gears from big/big to small/small without folding in on itself in the latter. Figure out the wrap requirement of your system this way: Wrap requirement = ( [biggest chainring - smallest chainring] + [biggest cog - smallest cog) ].

This number should be equal to or less than the RD's rating. On a triple, you can exceed this to some degree since you shouldn't be in the smallest chainring and several smallest cogs. But for a double, I'd want to at least be able to use the second-smallest cog (if not the smallest, on occasion) while in the smallest chainring.