What are sources (in the poskim) that say l'chatchila one can daven shachrit until chatzos? (if there are)
To the best of my knowledge I am not aware of any source L'chatchila. Here is the source for B'Dieved.
Orach Chaim 89:1 says that if one missed Davening Shacharis within the first four hours of the day they can B'Dieved Daven until Chatzos. Mishna Berura explains that this may be done even if it was intentional.
However one who Davens after the first four hours one should not say the Brachos of Kriyas Shema (Yotzer Or, Ahava Raba, Go'al Yisrael) Orach Chaim 58:6
It's unclear what you mean by l'chatchila. For the purpose of this answer, I will assume you meant:
If I haven't davened yet, am I still permitted to daven?
One version in the Tur (OC 89) is understood by the Taz (OC 89:1), and Shulchan Aruch HaRav1 (OC 89:2) rules that way as well, that if one hasn't davened within the first four hours of the day, he may still daven "l'chatchila" until midday.
The Derech Chaim (28:2) writes this as well.
1 According to the footnotes there, the Bach also holds that way, but I didn't notice it when I skimmed through it.