I am not sure a greaseable bearing will fix this. Sure it seems sensible to suggest if it had grease it would not fail.
But, why is it failing? If it is failing because the bearing is not able to take the load being applied on it, then all the grease in the world will not save it.
If it is failing because of lack of grease, where is the grease that is put inside it when it is made going? A sealed bearing will run a long time before wearing out, because the grease is sealed inside.
If it is failing because the grease was washed out of the bearing by use of the WD40, the solution is obvious. I would assume though, the WD40 was being used because at least one bearing had failed, without having the WD40 put inside.
Is it failing because the bearings are Chinese junk, and defective? Then possibly the solution is to replace them with nonjunk.
Can the bearings be removed, (pressed out), of one of the old pulleys? and can they be dissembled to do a postmortem?
If it is removable, there is usually a number on them that you can use to buy a replacement bearing.
If they are not removable, do you have a surplus store in your area or a mower shop that has a lot of stuff like this, perhaps you could find an alternate replacement?
A larger bearing would likely be a solution if you can find such a pulley and modify your deck to use it.