I have the exact same cotter pin assortment that I got from HF for less than $5 -- if you use a 20% off coupon it will be less than $4.
You don't need stainless for this application, the pins will be constantly bathed with oil.
When I wrote my initial response, I was thinking that it would be best to go with stock, just in case there is something extra-special about the stock cotter pins. This is not a job that you want to have to re-do. But, on further reflection, I think any cotter pin would be okay in this case. The cotter pin is only keeping a very tiny sleeve from falling out of a hole that it is in. The sleeve really doesn't have any forces on it, and almost anything would keep that sleeve from falling out IMHO.