Bekker Import carries them

If you are talking a USA legal conversion, be ready for a large tab and some
angst.  The entire light bucket must be replaced (not a task for the feint
of heart), and then you'll have to deal with will the monitoring interface
(will they be correctly monitored or indicate a lamp malfunction?!?) Cost is
likely to be around $1.5 to $2K for the parts, and mucho more dinero for the
installation.

Illegally, one could merely plonk in a conversion kit that uses the existing
headlamp assembly (ballasts, igniters, D2R/D2S lamps, harnesses, etc would
be included in the kit). Unless you got creative, you'd have no place to
mount the hi-beam lamps in a non-xenon (R129) SL500 headlamp assembly, beam
focus/collimation would be incorrect, and you'd not meet leveling or other
requirements.

If you're concerned with illumination, the stock halogen lamps are not bad.
If you want a tad more intensity, some aftermarket replacement halogens will
do that without creating a "lamp malfunction" indication. Some are even
available which give the bluish tint people associate with HID lamps
(actually, a blue or purple tint is more common with HID setups that use
projector lens... the cheap single-element lenses used cause chromatic
aberration. Mercedes use a reflector system producing white light when they
reach operating level [no chromatic aberration]. As the lamps age, the
spectrum shifts to become more bluish).

FWIW, I have a CLK with xenons and love 'em, and though not afraid to tackle
most projects, have no intentions of modifying my SL500. Give the stock
lamps a chance, you just might enjoy this fine automobile the way God
designed and Mercedes built it! (What?!? Was I outta' line with that
comment? :o)

Cheers,

Paul

PS: Food for thought from  http://faqlight.carpassion.info/

Snip--->

What kind of headlamps are legal to use?

Retrofitting of an ENTIRE sealed OEM HID headlamp from a different vehicle
keeping all original functions* intact is legal in North America. Obviously,
it must be mounted in the approximate same height and as solid as on the
original vehicle. In a court case worst scenario, it will most likely will
be used against you, but the right lawyer will be able to defend you.

*Note that functions consists of low beam, high beam, markers, turn signals,
amber side reflector, leveling system (if equipped), cleaning system (if
equipped) and AFS system (if equipped).
All other kinds of retrofits are illegal, unless results from a testing lab
can show otherwise. The cost of such test is around $10,000 and consists of
various environmental, durability, material, aimiability, and so on. Even if
you are willing to pay this amount, it is not granted that you will pass.
Based on the report you will have to make design adjustments where it failed
and go for another test. All in all, designing legal headlamps is not for
the DYI mechanic.

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