I'm trying to improve my upper position cello technique. Does anyone know of some good cello resources that are baroque in nature?
"Upper position" and "baroque in nature" seem like somewhat contradictory requirements.
At any rate, the Bach Cello Suites are not likely to be mastered and put away soon and they are pretty baroque.
Bach has the advantage of writing for rather than against the instrument, so while the solo string pieces are really tough, they are also immensively satisfying musically.
The downside, of course, is that you cannot expect to play them transposed or in a substantially different position for practice purposes. They just stop working then.
Do you have a gut-string Baroque cello and a Baroque bow, or are you playing a modern steel-string cello with a modern bow?
For historically-informed, period-correct playing with a Baroque bow, check out publications by Baroque string instrument educator Martha Bishop, who lives in Atlanta, Georgia, USA. These resources may be worth checking out even if you have a modern instrument and bow.
Disclosure: She is a friend of mine.
If you want to work out in the upper register in general, there are plenty of 19-th century studies (Duport, Popper and Piatti come to mind, implying use of the thumb positions), but as others have pointed out, most baroque composers who wrote cello parts or works did not go very high compared to modern cello works. If you are really interested in Baroque music, you should consider working on your bowing, the execution and improvisation of ornaments, the treatment of "notes inégales" (the Baroque "swing"), less on the upper register per se. Consider finding a violinist to work with to study the many sonata for two instruments of baroque composers.
Concerning Baroque and pre-Baroque with a few high notes, you might look for transcriptions of works for viola da gamba. Caix d'Herveloix and Marin Marais are important examples. They usually go higher than most, but usually not higher than c5 (say 3rd finger, 7th position, 1st string).
When you speak of upper register, you do not mention what is your current range. Assuming you have a modern cello with steel strings, if you read fluently the G/soprano clef on your cello up to a5 or b5 (a fifth above the 1st string of the violin which is tuned open as e5), why don't you try simple or simplified baroque works for violin or viola ? They will be a challenge to make sound good and play in tempo. Try to avoid vibrato by default. It is easy to find scores and they will be of baroque origin, if not of authentic character in their execution.
I'm not particularly sure if this qualifies as baroque, but it definitely strikes me as baroque-like in style: I have a book of etudes called "Modern Method for the Violoncello" by Stephen De'ak. It's Volume II of a three book series that gradually eases the player into thumb position through etudes of a pretty challenging nature (although this is subjective, of course). The book has a guide to thumb position, with repetitive exercises followed by etudes. It also includes practices for some more advanced techniques as well, and all scales (major, natural and harmonic minor) and arpeggios.
I use this book as an intermediate cellist with guidance from a teacher at the New England Conservatory; it's not only helped my thumb position development immensely, but also my sight reading, intonation, and overall dexterity (as it should, they're etudes after all).
A wonderful collection of pieces from the Early Baroque era are those By Dominico Gabrielli. Written about 1680 in Bologna they are some of the earliest masterworks for the cello. Among the various sonatas with accompaniment and canons, the collection includes 7 Ricercars for cello solo which explore the same ideas found in Bach's suites but in a less complex style. Interestingly they also occasionally incorporate the detuned 'a' string, brought down to a 'g' as Bach would later do in his 5th suite.
You can try Duport's exercises, Boccherini's sonatas or even french baroque sonatas that were written both for cello or violin.
If you are willing to play baroque cello, with gut strings and a copy of a historical bow, I recommend you to look for Tartini's letter for Maddalena Lombardini.
All these resources are on imslp.org.
Hope it helps!