I saw in Rav Steinsaltz's commentary on Sanhedrin 17a a reference to Tosafos Rabbeinu Peretz. Is Tosafos Rabbeinu Peretz on Sanhedrin available online anywhere?
I have not been able to locate any Tosafos Rabbeinu Peretz on Sanhedrin. Indeed, for what it's worth, Wikipedia reports that we don't have his comments on Sanhedrin.
However, thanks to @Dov, I was able to locate the existence of Tosafos Rabbeinu Peretz HaShalem. There, the book description states that they have printed from manuscripts the comments of Rabbeinu Peretz on other masechtos than what was previously available. They write that for Sanhedrin, they collected comments from the sefer Chamra VeChayii, written by Rav Chaim Benveniste, a 17th century Turkish Rav.
His sefer seems to be a Shita Mekubetzes style work, collecting the insights of Rishonim on Sanhedrin. It's not a book dedicated to Rabbeinu Peretz. Indeed, a search on Otzar HaChochmah yielded only 16 references to his comments in the entire work. Furthermore, they're only attributed to the students of Rabbeinu Peretz.
I assume this is what Rav Steinsaltz zt"l is referring to. If so, the two comments of Tosafos Talmidei Rabbeinu Peretz on 17a can be read here (bottom left, ד"ה אלא) and here (top right, ד"ה נגבית). If you have access to Otzar HaChochmah, an easier to read print of these comments can be viewed here.
On the Otzar HaChochma website you can access the Oz Vehadar Murchevet edition of Sanhedrin here which has the Tosafos Rabbeinu Peretz at the back (as detailed on page 8, second column on the left, first bold line) - however as it is beyond the free allowance you would have to pay to access the whole sefer.