For about $314 + tax and shipping, KYBs struts are available for your Rx300
2003 LEXUS RX300 3.0L V6 Strut | RockAuto
Labor should be about $900 for a total of $1200 or so if you buy the struts.
To get a labor estimate, enter your vehicle and required service information and zip code at:
www.yourmechanic.com
They break down the parts and labor costs. I don't know which brand of parts they use but if you were to use their service, you can ask that and whether they would allow you to supply the parts.