I am needing a door lock actuator (passenger door) for my 2000 Lexus RX300. Do you know of any parts places besides Lexus where I might find one ?

Try to find your solution here, from Germany Lexus Owners:

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I justreplaced my door lock actuator on my 2003 Lexus RX300. A rather comprehensive set of instructions for removal of the door panel can be found at: clublexus.com door+panel+removal

To remove the door handle trim: after removing the screw in the door handle trim, pry (I use a painters 5 in 1 tool) the trim piece outwardsfrom the rear of the door panel at both the top and bottom of the trim piece. This will release the two small rear clips. Then push the trim forward to release the main clip located on the back side the handle trim piece

As the least expensive OEM replacement door lock actuator for Lexus RX300 is $230 or more, I opted for the $15 replacement option as outlined below: us.lexusownersclub.com/forums

Keep in mind that this job outlined above is a bit on the technical side.If you aren't that talented, spend the money and order a OEM Lexus replacement latch actuator and do the installation as the OEM latch assembly is not repairable.

Viel Gluck (good luck in German)
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I hope helps with this. Good luck.