I also got my knock sensors from subaruparts.com... I think it was about $40 each or something like that. Anyway, I'd just buy a pair of new ones and do it yourself.
You can do it without removing the intake plenum, but you need small, steady hands and a small 10mm wrench. You also need the patience of a saint. I did it, but the rear knock sensor was a bear to get back in; getting it to stand upright, get the bolt into the threads and tighten it down one-handed in VERY confined spaces was tough, but it's doable. I also have skinny hands
Note that to get the sensor out with the 10mm wrench, you'll maybe be able to turn the bolt 1/4 of a turn then have to reposition the wrench with very limited headroom. You'll also maybe get the bolt out 2/3 of the way before you can no longer get the wrench in there... then it's fingertips twirling the bolt until it pops off.
Took me about 90 minutes in total to replace both through the opening after removing the alternator. You might get more clearance if you remove the A/C compressor and move it to the side as well, but I didn't want to mess with all those aluminum hoses!