Capacitors can be purchased from many sellers.
I usually buy them from aliexpress, or ebay, and otherwise digikey.
Usually I choose the Rubycon brand, or other well-known brands such as Philips and Panasonic or sometimes the cheaper one.

Concerning your problem, if the replacement of the psu and whether the caps are replaced does not help, then it may be the motherboard.
Then it becomes difficult because it can be caused by various things.
One is the nand chip, the memory block, or worse the cpu, possibly bad outdated solder.

I think it's best to try using a terminal program like putty in rs232 serial mode to attempt to see when/why it goes wrong.
For that you need an rs232 db9 null modem cable, an rs232 usb to db9 serial cable is also possible.