Irrispective of the tuning you decide for, there seems to be an obligatory recommendation for the 7th string, the bronze wound Aranjuez 7th specially developped for Yepes. Slightly thinner than silver plated copper 7ths, very flexible and can be tuned from E down to A'. Just out of curiosity I used this string for all 4 extra basses in diatonic, romantic tuning and this worked quite well. BTW it's a good visual aid, where the extra basses start and this string is comparatively cheap!
For the rest it depends on the tuning you decide for. For romantic tuning (D, C, B', A') you will need thicker basses available from La Bella (10PH), Hannabach (815) or Pyramid just to name a few. BTW La Bella 10(PH) is the most economic solution!
For Yepes tuning you can rely on either Aranjuez' extension set with bronze winding including the above mentioned 7th or as the 7th string at D is the lowest note with this tuning, you can fill up the other basses with regular strings of appropriate tension! That's probably the reason why no more Yepes bass extension sets are available!
If I remember well I used Hannabach bass strings for this!
8: A# - A (815 LT), 9: G# - A (815 HT), 10: F# - E (815 SLT)