@PedroTMelo 

It's amazing how no one can answer a question as simple as the one I asked, not even TP-Link technicians answer !!!
Unfortunate.

Pedro Melo

@PedroTMelo maybe if you said what country you were in someone could help you?

For what it is worth, I am in the UK, and I don't see the single devices available for sale anywhere. I ended up buying one of the lower priced Kits so I could extend my network without having to pay full whack for the higher end ones.

@dunxd 

Sorry, I live in Portugal.
In fact it is stupid to want to extend our network and not be able to buy the receiver without having to purchase a complete Kit. It doesn't make any sense, I don't understand TP-Link's policy.

@PedroTMelo I'm not sure it is TP-Link's fault. They offer the devices, but I haven't found a retailer that carries them.  Have you tried asking TP-Link which retailers in Portugal carry the devices you want to buy? Or perhaps another country in the EU.

@dunxd I didn't ask Tp-Link, but I already looked on Amazon, e-Bay and other online stores and I didn't find it, even on the Tp-Link website I can't find it except in Kit. It's a pity that brands don't sell their own products , made life much easier for consumers.
I will contact them to see if they tell me where I can buy.

@PedroTMelo Agreed.. This has made the life easy for the consumers to a great extent. 

@PedroTMelo I did notice that the German version of the site has links to buy individual TL-WPA8630P devices at Mercateo, and in the UK to Amazon.  However, Amazon (and it's resellers) don't have any in stock.  I guess demand for the individual models isn't big enough for retailers to carry them, which is disappointing.  I bought three kits so far, so have two TL-PA8010P that were excess to requirements.  I have subsequently connected a Raspberry Pi using one of them, and realised I could put my printer somewhere more convenient using the other.

If I had a choice, I would have bought individual TL-WPA6630Ps after the first KIT purchase.

I see that it is possible to buy the TL-WPA4220 individually on Amazon UK, but it only costs £2 less than the Kit version from Amazon UK.