Slide-Out Seals - Factory RV Surplus
and http://www.all-rite.com/rubber-seals-gaskets-c-38.html
works for me..sometimes...I have ordered a few seals for my 10 year old CKTS.
I have another link with a bigger selection but having a hard time finding it...they had quite a bit more selection.
Also believe it or not Ebay...just search RV slide out seal..you will find yours usually. Find more there than RV online suppliers.