There are "some" parts available. They usually have the power supply, fan unit, and optical drive. They may also have the pogo board, bezel, hinges, but these are more limited. They also have the 6TB drives. They try to maintain some parts for these servers, and will continue to do so into the future provided the cost of parts is reasonable and it makes business sense to keep these in inventory (basically they have to balance the order requirements when purchasing parts against actual demand/need for the parts).
NOTE: One important caveat regarding power supplies. K has gotten "burned" on a few returns of power supplies for Servers after owners purchased the PSU hoping it would fix their problem, and then when it didn't they insisted on returning the PSU claiming they were told the PSU was needed to fix the problem. Because of this, there's now an added step required to confirm owners are aware that replacing the PSU may or may not fix their problem, and the PSU's are NOT returnable.
I'm not aware of any 3rd party repair sites.
Yes, there are trade-in and trade-up promos for older Premiere components. There are currently two programs, one for trading Premiere to Premiere, and one for trading up from Premiere to Strato/Terra. Your dealer will have the specific details and prices. You can do this even with some models that are not currently working. Again, your dealer can help with this, but if they're not responsive reach out to me by email or PM and I'll explain the details.
Jim