Contact the church they should have records of the purchase. There are probably no deeds or receipts. When my parents bought a plot years ago they got a 'Cloakroom (raffle) ticket', we were told several times over the years of location and importance of said ticket.
It was always wondered about the legitimacy of this ticket/plot, as it turned out the priest had sold them to raise money for his own purposes. More recently a relative died and to cut a long story short my parents offered the family the plot. Enquiries were made by the undertakers and it turned out the church did actually have a record and the special Cloakroom ticket was never needed.
It depends on the graveyard, some are dealt with by the local county council, some privately by the church. If you contact the church, county council or a local undertaker they should be able to tell you who keeps the records/sells the spaces