The BMS offers funding, both for Phase I (Master) and for Phase II (PhD) students.
The application for a scholarship can be included in the application for admission, there is a special section - no extra application materials are needed.

All BMS scholarships are tax-free and do not include health insurance.
In addition to the scholarship, one can earn up to 6,000€ per calendar year without paying taxes.
Phase I students can find a tutor job or student asstistant position at the Berlin universities to supplement their living expenses.
Phase II scholarship recipients should talk with their supervisor and mentor regarding teaching/ tutor positions to supplement their living expenses.

Phase I scholarships

Typical Phase I scholarships are granted for 1+18 months (extendible by 6 months), and amount to 1,000€ per month.

Students in Phase I with sufficient German language skills are normally expected to support themselves by other means, either with student jobs like tutoring for undergraduate classes, or by applying for outside scholarships, for instance from the Studienstiftung des deutschen Volkes.

Phase II Funding

BMS Phase II scholarships are granted for 24 months (extendible by 12 months), and amount to 1,600€ per month, tax-free (no insurances included). There are extra child-care funds reserved for BMS students who take care of children.
The BMS also offers project positions (75% TVL13) for three years in certain cases.
Applications for Phase II received via the BMS application portal and requiring PhD funding are sent to potential supervisors within the BMS faculty and considered for any open PhD positions in the supervisor's research goup.

Approximately 20% of Phase II students will receive BMS funding. Funding for all other Phase II students will come from MATH+ projects, Certified Units, from third-party funding available to individual professors or from the departments’ budgets. All Phase II students should take advantage of the many opportunities offered by the mathematics research environment in Berlin, which includes:
• (International) Research Training Groups (RTG)
• International Max Planck Research Schools (IMPRS)
• DFG Collaborative Research Centers (CRC/ SFB)
• ECMath
• Zuse Institute Berlin (ZIB)
• Weierstrass Institute for Applied Analysis and Stochastics (WIAS).

A complete list can be found here: www.math-berlin.de/about-bms/people-a-organization/units
The BMS integrates the above projects and institutions, as well as many others, as Certified Units, Associated Units, Cooperating Research Centers and Research Institutes. They guarantee PhD supervisors and suitable research environments for all Phase II students.

Other scholarships

Other scholarships may be available from funding agencies such as
German Academic Exchange Service (DAAD)
Stibet (DAAD)
Studienstiftung des Deutschen Volkes
FAZIT Stiftung (in German)
via exchange programs of the universities (for example, at TU Berlin)
via teaching assistantships (for example, at TU Berlin)
Eva-Wolzendorf-Stipendium (in German, at FU)
Berlin Funding for Graduates (Elsa-Neumann-Stipendium des Landes Berlin)

HU master students can apply for the Humboldt Research Track Scholarships: https://www.humboldt-graduate-school.de/en/promotionsinteressierte-en/uebergangsstipendien/allgemeines

for women:
The Caroline von Humboldt Grant Programme (at HU Berlin)
Degree completion grants from the Council of the Central Office for Women's Affairs "Promotionsabschlussstipendium (PAS) (at TU Berlin)
Personal financial support (at FU Berlin)

An overview of additional foundations supporting PhD students can be found here:
Center for Junior Scholars (TU Berlin)
Begabtenfoerderungswerke
StipendiumPlus
Research in Germany
Funding opportunities for graduates -- Promotionsportal of FU Berlin

Other Funding opportunities:
Berlin University Alliance - Job Portal
Physikalisch-Technische Bundesanstalt