http://www.jeep4x4center.com/cherokee/ei-xjbody-rear-parts.htm has them, although you need a parts catalog to figure out what is needed.
There are three retainer brackets (front, center, and rear) on each fender. The center retainer brackets are identical for each pair of fenders (so just one part number), while the front and rear retainer brackets are mirrors of each other (different part numbers for left or right). The front and rear fenders are also different, so they have different retainer brackets, and different part numbers.
Here are all the part numbers for my 91 XJ
Front Left
Front -- 5500 3232
Center -- 5500 3234
Rear -- 5500 3236
Front Right
Front -- 5500 3233
Center -- 5500 3234
Rear -- 5500 3237
Rear Left
Front -- 5500 3238
Center -- 5500 3240
Rear -- 5500 3242
Rear Right
Front -- 5500 3239
Center -- 5500 3240
Rear -- 5500 3243
$71.88 for all of them at Jeep 4x4 Center, before shipping.