Can we ask for help from sinless persons except Allah? But one thing should be remembered that we are not consider them as Allah, we are considering them as source through which Allah help us?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Al-Istighatha means an appeal for help in case of distress. It is of two kinds: (1) al-Istighatha with The Creator and (2) al-Istighatha with creatures.
As for the first kind, it is not only legal for one who is in distress to seek Allah's help since He is The Only One who can relieve sorrows whatever they are, but also it is a duty.
Allah Says (interpretation of meaning): {And your Lord said: "Invoke Me, [i.e. believe in My Oneness (Islamic Monotheism) and ask Me for anything] I will respond to your (invocation). Verily! Those who scorn My worship [i.e. do not invoke Me, and do not believe in My Oneness, (Islamic Monotheism)] they will surely enter Hell in humiliation!"} [40: 60].
Al-Istighatha is a high form of Du'a (supplication) as stated in the Qur'anic verse: {(Remember) when you sought help of your Lord and He answered you …} [8: 9]. He also Says: {… and He answered you …} [8: 9].
As for al-Istighatha with creatures, it is also of two types:
The first type is al-Istighatha with some creature for something that he can do. This type is good since Allah, The Great and Almighty, told us about a similar incident in the story of Prophet Moses (Alaihi al-Salaam) {… The man of his (own) party asked him for help against his foe, so Musa (Moses) struck him with his fist and killed him. …} [28: 15].
So, one who is in distress can seek help from another one to push away an enemy or so on.
The second type is al-Istighatha with some creature for something that he cannot do such as seeking help for providing with progeny, raising from the dead or relieving sorrows and so on.
Such an Istighatha constitutes a form of Shirk (polytheism) as stated by Muslim scholars. Since the one who seeks al-Istighatha thinks that this creature can do what the Creator does. Such a situation is completely forbidden and a form of Shirk even if the one whom we seek his help is an angel, Prophet (Sallallahu Alaihi wa Sallam) or a righteous man. If one believes that those are no more than means, he also comes under the same Shari'a ruling, i.e. a form of Shirk.
Allah states in the Qur'an that those who take idols as a means between them and The Creator as Mushrik people (polytheists) and orders Muslims to wage war against them.
{… And those who take Auliya' (protectors, helpers, lords, gods) besides Him (say): "We worship them only that they may bring us near to Allah". …} [39: 3]. He also Says: {…, and they say: "These are our intercessors with Allah". …} [10: 18].
Allah knows best.