They're about £30 from your dealer. RealOEM.com BMW R53 Cooper S Timing and valve train-timing chain
Down back of engine and a 19mm offset ring spanner is your best bet for getting it out.
Can be done from working above or remove O/S wheel and arch liner.
I'd recommend changing the oil and flushing engine too.
The new one comes compressed and once fitted the oil pressure extends it forward and tensions the chain