NorthwestTechnical carries some parts, but did not have any listing for the 4100 sleds. They had 4000, but they are different. If you have one you may see if they are interchangable. Since Adaptec has discontiuned this model now, they will likely sell of their inventory. They did this with the other models.
If you are NOT going to moving the 4100 around you can have them lay loose. Or at least install some screws to elevate the drives for better cooling.
Or search eBay for them. Someone may have a failed unit they are parting out.