Where can I find an English translation of the Buddhavatamasaka Sutra? I just finished re-watching this lecture by Jan Nattier, an academic expert in Mahayhana Buddhism. She mentioned that the 10 Bhumis in the 26th chapter of the Avatamsaka had a predecessor called the Buddavatamasaka which had a more coherent enumeration of 10 Bhumis and had steps that mere mortals could follow.

I can't find it it anywhere. Wikipedia does mention the Daśabhūmika Sūtra, but that appears to be the free standing later version of the 10 Bhumis.

The Cleary translation of the Avataṃsaka Sūtra is the later version of the 10 Bhumis that is mostly opaque literary bombast, describes the results of the steps but not the steps and other than bodhicitta, doesn't describe any steps that mere mortals can engage in in this life.
To the best of my knowledge there is no other translation of the Avataṃsaka Sūtra. 

As far as text called Buddhāvataṃsaka - there is nothing in the Digital Sanskrit Buddhist Canon; nor in GRETIL - Göttingen Register of Electronic Texts in Indian Languages.

This suggests that no Sanskrit manuscript survives. Searching for a Sanskrit name in the Chinese Canon is not quite impossible, but very very difficult.

Responding to the comment:   

Taisho Edition of the Chinese Tripiṭaka (with links to the online CBETA version):  


T280 &#20315;&#35498;&#20828;&#27801;&#32147;; Scripture on the Original Deeds of the Bodhisattva as Explained by the Buddha (Ch 7 &amp; 8 of Avataṃsaka)  
T282 &#20315;&#35498;&#33769;&#34217;&#27714;&#20315;&#26412;&#26989;&#32147;; Original Deeds of Bodhisattvas Seeking Buddhahood. (Ch 11)  
T282 &#33769;&#34217;&#21313;&#20303;&#34892;&#36947;&#21697;; Book on Bodhisattvas’ Ten Abodes in the Practice of the Way (Ch 15)   


But these are not all attributed to Lokakṣema. The last looks most promising, but is attributed to Dharmarakṣa in some sources. However see Nattier: A Guide to the Earliest Chinese Buddhist Translations: Texts from the Eastern Han and Three Kingdoms Periods. This article questions the traditional attributions and assigns T280, T282 and T283 to Lokakṣema. The last looks most promising as a source for the Daśabhūmi or 10 Levels, but I cannot see any English translations online. Unfortunately very little of the Chinese Tripiṭaka is yet available in English. Though there is a Numata Foundation project under way to translate it all, it will take decades.