Agreed! 
Sister Honda Genuine products are best, although costly, but you need not buy them from 3S.
As far as their purchase is concerned, you can always buy from 1S dealerships (Details at https://www.honda.com.pk/index.php/dealer-network/#1570477309850-bffa3c42-c37c and https://www.honda.com.pk/index.php/contact-us/1s-dealers-spare-parts/ )
Then, there are reputed and respected shops in the markets, which sell the same, authentic and genuine products at quite reasonable rates. You need to find them out.
I am based in twin cities, so unfortunately, I regret that I cannot guide you about any such shop / market in Karachi. Let experts / members from Karachi guide you best in this regard.
If one is not an expert, then he / she needs to be very careful, as some shops might sell fake products, in the name / price of "Cheap'', or authentic products.
It all depends on you. If you can afford, then Honda Genuine is the Best, else if, aftermarket is also fair.