>> if your GPU luckily isn't TeraScale2 but GCN1 [...] then [...] you can
>> keep this card for OS4 in the primary slot and just need to add to
>> another slot a card 3D-supported by MorphOS and Linux, which would
>> be R5xx-based, as has been mentioned numerous times in this thread.

> May I go to buy a R5xxx one?

R5xx (not R5xxx), which is anything from RV515 (Radeon X1300) to R580+ (Radeon X1950 XT(X)), see https://en.wikipedia.org/wiki/Radeon_X1000_series#Chipset_table.

> Where can I do it?

As those cards were released between 2005 and 2007, you will get them NOS at best (but still rarely). It's easier to get them used, so look for them where used stuff is being sold.

> is this suitable and compatible with my AmigaOS 4.1,
> Morphos and of course Linux/Fienix?

As I wrote, the R5xx-based card is for MorphOS and Linux. That's why you'll have to keep your GCN1-based card in the primary slot for OS4, as I also wrote.