How available are this models parts and do I have to get it serviced at a registered jaguar dealer?

I have a 90 and 92 and have not found any parts problem availability.
Ebay has many new and used parts, and a number of aftermarket suppliers who have Jag parts, both new and new by other parts suppliers.
Your biggest problem may be to find somebody who can and will work on the car. They will want to buy new Jag parts which are very expensive.
If you can a British car shop, you will likely find a mechanic or two who can work on the car but they will want to use Jag parts.
I have a semi-retired guy who works on the road when I have an issue and he will use the parts I get, as long as they work.
A good manual and tools should be part of what they supply with these cars. They are great but need care and attention or very expensive problems occur.