spool cover performance pwr400dgta?

i have a performance power electric grass trimmer model PWR400DGTA and all it needs is a spool cover, B&Q sell the spools but no replacement covers,i have tried almost every spare parts company an no luck so far, this is annoying me that i may have to throw out a really good strimmer for the sake of a bit of plastic
why do b&q sell these items when they know they'll only have a short life.