Most of extenders comes from Hong Kong. If you look for european seller, the only advantage is shipping delay. The price is obviously better in a Hong Kong shop.
That in mind, I bought a dozen cable at Camera2000, since shipping is free and prices are low. ETA was 2-3 weeks, I was lucky as cables were at my doorstep in about 10 days.
In order to save some time before the arrival of the cables from previous shop, I ordered half dozen cable at this eBay shop located in the UK. Same cable, same quality, twice the cost, half shipping delay.

Time is the only real factor !