This might be somewhat scattershot, I'm moving and many of my books are packed away.

What texts or evidence showed the practices of these soothsayers/Seiðmenn/witches and their hunters?

It's worth noting that belief in witchcraft was officially condemned a number of times by the Church, so they did not dedicate much effort to explicating the actual nature of these practices. Those who believed themselves to be witches were classified as deluded, guilty of heresy rather than actual sorcery. Since witches didn't exist, hunting them was certainly not a legitimate enterprise. In some cases, cf. the Council of Paderborn, those who killed supposed witches were condemned to death (in this case it seems that the "witch hunting" itself may have been a pagan practice involving cannibalism). Even later, during the time of the Inquisition, when witchcraft beliefs were encountered the inquisitors don't really seem to know what to do with them. They often simply sent them home with a vague conclusion that these beliefs were the result of delusions and that they should be generally discouraged, but without further punishment (cf. Ginzburg's The Night Battles, concerning the belief that people were able to fly while sleeping to engage in spectacular battles with witches over the fate of the harvest).

At the same time, these condemnations give us some idea of what people thought that witchcraft looked like. One of the most commonly condemned beliefs, especially during the Carolingian period, was in "storm-callers" who could call down violent hail which would destroy your crops unless you paid them off. It's fairly obvious why the Church would want to crack down on this sort of magical protection racket. Also condemned is the idea that during a lunar eclipse it was necessary to make a loud racket to drive off evil spirits and to allow the moon to re-emerge.

The Canon Episcopi, a famous condemnation of belief in witchcraft of the 11th century, condemns the belief of "certain wicked women" who had been deceived by Satan into believing that they had entered into the service of the goddess Diana and would fly on beasts at night to do battle on her behalf (similar to the beliefs Ginzburg discusses that I mentioned above). Priests were told in no uncertain terms to debunk such beliefs as caused by evil spirits. So, these beliefs are definitely "out there" and the Church is actively working to stamp them out all the time.

Valerie Flint has written a good bit on this subject in The Rise of Magic in Early Medieval Europe and Aron Gurevich's Medieval Popular Culture is also a good source.

Did christians ever seek the counsil of pagan wise men and did they still believe in the power of germanic or celtic gods despite worshipping christ?

Probably, human nature makes it unlikely that cultures will simply abandon centuries of tradition because the local chief told everyone that they're Christians now. We can see some traces of this in hagiography, for example St. Benedict destroyed an actively used temple to Apollo on Monte Cassino before constructing his monastery there. There are also instances where saints confront rival wonderworkers which seem to be remnants of the pagan religious structure.

Another good example is in Beowulf, when Hrothgar's men offer pagan sacrifices in a desperate attempt to prevent the Grendel's depredations:

Whiles they vowed in their heathen fanes

altar-offerings, asked with words

that the slayer-of-souls would succor give them

for the pain of their people. Their practice this,

their heathen hope; 'twas Hell they thought of

in mood of their mind.