Most of the saints or rishis we see are male. Are there any female saints (except devotees through Bhakti yoga)?
We see that many females also follow their husbands, serve them and partake in their tapasya. In Srimad Bhagavatam When Dhrtarastra and Gandhari are missing from the palace Narada Muni tells this to Maharaja Yudhisitira:
SB 1.13.53 — On the banks at Saptasrota, Dhrtarastra is now engaged in
beginning astanga-yoga by bathing three times daily, in the morning,
noon and evening, by performing the Agni-hotra sacrifice with fire and
by drinking only water. This helps one control the mind and the senses
and frees one completely from thoughts of familial affection.
SB 1.13.57 — O King, he will quit his body, most probably on the fifth
day from today. And his body will turn to ashes.
SB 1.13.58 — While outside observing her husband, who will burn in the
fire of mystic power along with his thatched cottage, his chaste wife
will enter the fire with rapt attention.
Similarly we also see this in this case of Arci, wife of Maharaja Prthu (incarnation of Vishnu) also:
SB 4.23.19 — The Queen, the wife of Prthu Maharaja, whose name was
Arci, followed her husband into the forest. Since she was a queen, her
body was very delicate. Although she did not deserve to live in the
forest, she voluntarily touched her lotus feet to the ground.
SB 4.23.20 — Although she was not accustomed to such difficulties, Queen Arci followed her husband in the regulative principles of living in
the forest like great sages. She lay down on the ground and ate only
fruits, flowers and leaves, and because she was not fit for these
activities, she became frail and thin. Yet because of the pleasure she
derived in serving her husband, she did not feel any difficulties.
SB 4.23.21 — When Queen Arci saw that her husband, who had been so merciful to her and the earth, no longer showed symptoms of life, she
lamented for a little while and then built a fiery pyre on top of a
hill and placed the body of her husband on it.
SB 4.23.22 — After
this, the Queen executed the necessary funerary functions and offered
oblations of water. After bathing in the river, she offered obeisances
to various demigods situated in the sky in the different planetary
systems. She then circumambulated the fire and, while thinking of the
lotus feet of her husband, entered its flames.
SB 4.23.23 — After
observing this brave act performed by the chaste wife Arci, the wife
of the great King Prthu, many thousands of the wives of the demigods,
along with their husbands, offered prayers to the Queen, for they were
very much satisfied.
Devahuti, Lord Kapila's mother also performed tapasya as explained here
Else where we also hear about the austerities of mother Vedavati, mother Parvati and so on.
Why do we find female rishis only rarely? Is there any reason?
Generally women are considered to be like children who need protection. So they are with father in their child hood, husband in their married life and with children when their husband takes sannyasa and leave home. In some cases as above we see that women follow their husbands for tapasya and give up their body with them. In those cases they go where ever their husbands go. Chaste women share the results of their husbands' austerities.
Through which yoga females can get moksha? Can they get moksha using yogas other than Karma yoga and Bhakti yoga?
While in previous yugas they shared their husbands path, as above, in kali-yuga the only effective ways is by chanting the holy names of Hari (bhakti yoga). Whether men or women this is the most effective way. Below is a verse from Brhan - Naradiya Purana as cited by Sri Caitanya Mahaprabhu.
harer nama harer nama
harer namaiva kevalam
kalau nasty eva nasty eva
nasty eva gatir anyatha
“ ‘For spiritual progress in this Age of Kali, there is no
alternative, there is no alternative, there is no alternative to the
holy name, the holy name, the holy name of the Lord.’ Caitanya
Caritamrta Adi 7.76 (http://www.vedabase.com/en/cc/adi/7/76)
Is self-realization for women easier when compared to men?
Not necessarily, regardless of who it is if they surrender to Krishna they can attain perfection.
Bg 9.32 (http://www.vedabase.com/en/bg/9/32) — O son of Prtha, those who take shelter in Me, though they be of lower birth – women, vaisyas [merchants] and sudras [workers] – can attain the supreme destination.
One thing to note is generally women have softer nature, so those who take advantage of it can surrender little easily. My observation is that more women in India do pujas than men but at the same time they may not be as serious as the men who do take up some spiritual path. So it all depends on individual's sincerity.