What is the basic difference between sAdhArA and nirAdhArA diksHA as per the scriptures?
In one of the comments, you said that Niradhara=without rituals and Sadhara=with rituals.
Then, Sadhara is clearly what is called the KriyAvati DikshA in KulArnava Tantram. Niradhara can be of three types as mentioned below.
In KulArnava Tantram's 14th Chapter, various types of initiations are described. Some of which are based on rituals where as the others are free from any such requirements.
The VAg, Drik and MAnas DikshAs are stated to be performed without any rituals.
SparshAkhyA devi drik-sangyA mAnasAkhyA maheswari|
KriyAyAs-Adi-rahitA devi dikshA tridhA smritA ||
O Maheswari -- Sparsha DikshA (which is accomplished by the Guru's touch), Drika DikshA (by vision) and MAnasa DikshA (by mind) -- these three are the types of initiations. In these initiations, no rituals and efforts are needed.
The nature of these three types of initiations are briefly given in the following verses:
YathA pakshi swapakshyAbhyAm shishun samvardhayecchanaih |
Sparshdikshopadeshashcha tAdrishah kathithah priye ||
Just like a bird gradually raises up the chicks under it's wings, know the Sparsha DikshA to be of a similar nature, O Beloved.
SwApatyAni yathA matshyo vikshanenaiva poshayet |
DrigbhyAm dikshopadeshashcha tAdrishah parameswari ||
O Parameswari, just like a fish raises up it's babies by simply vision, know the DikshA Upadesha called Drik to be of a similar nature.
YathA kurmah swatanayAn dhyAnmAtrena poshayet |
Vedhadikshopadeshashcha mAnasah syAt tathAvidhah ||
Just as a tortoise raises it's babies up by simply thinking about them, likewise the Upadesha called Vedha is entirely mental.
The MAnasa DikshA is also known as Vedha (or Vedhamayi) DikshA.
Now, some DikshA is based on rituals. This is called the KriyAvati DikshA.
DikshA mokshapradA devi saptadhA parikirtitA ||
KriyA, Varna, KalA, Sparsha, VAk, Drik and MAnasa DikshAs are the seven types of initiations that bestow Moksha, O Devi.
Among, these 7 types, KriyA which requires rituals, is again of 8 types:
KriyAdikshAshtadhA proktA kunda-mandapa-purvikA |
KalashAdisamAyuktA kartyavyA gurunA vahihi ||
The KriyAdikshA is again of eight types. In these initiations, Kunda (fire-pit in which Homa oblations are poured), Mandapa (canopy), Kalasa (a pot) etc are required. The Guru using the above means performs the (external) rituals.