I need to replace my converter and y-pipe (98 es300) and was wondering where I can get a factory part cheaper than the dealer, or if aftermarket is ok??

 

I'd run it aftermarket, the y-pipe you could get a muffler shop (not a chain place that charges $$$) to make a new y pipe (I assume you have some cracking or holes in the piping) and the converter, it's easier to go something like a Magnaflow. A 94005 model should fit fine on there, but the shop may know of a better number.