Walk of Fame Hostel and Samesun Venice Beach hostel are some of the cheapest place for accommodation. Can also try Samesun Venice Beach hostel.
As We Dwell is a fun co working space and a storefront at the same time, they sell stuff related to business. They have mail service, wheelchair facility and is pet friendly. Average internet speed is 15 mbps.
T-Mobile is a store where you can purchase a prepaid sim card. The staff is friendly and can help you with the installation.
The public transit system in Los Angeles is notoriously unreliable. Uber or renting a car are going to be better options, but pay attention to traffic patterns. Even if you know how to get somewhere, use a GPS, like Google Maps, since it can factor in traffic and accidents to find the best route. California drivers tend to be aggressive, so be careful. Don't text or call without a hands-free device since that is illegal in CA.