I bought the straight NGK ones around the middle of last year off internet. They made quite a difference on my tired bike.
I bought them online only a couple of quid each:
1x NGK Spark Plug Cap SD05F-B - 10-12mm
I bought them online only a couple of quid each:
1x NGK Spark Plug Cap SD05F-B - 10-12mm
-suck-squeeze-bang-blow-