Some art centers offer studios for rent on an hourly basis, and Craigslist and classified sections in newspapers list options as well. Art centers might also offer shared spaces for people willing to pay membership fees.
Art studios are often rented on a monthly basis, since artists typically need to store their work. Artists renting studios sometimes offer to share their space with other artists, and some might be willing to negotiate an hourly rate. Artist who plan to spend approximately the same amount of hours on their work each week may want to negotiate a weekly price, which might be lower than paying for the space by the hour.
Another option is to look for space that's not traditionally used for art. Some places might offer a practice space for bands that can be rented on an hourly basis. Photography studios tend to be expensive, but their bright, adjustable lighting makes them a great place to work on paintings and drawings. Warehouses and other commercial buildings often stay vacant for extended periods of time, so landlords might be willing to offer low rates and potentially storage. However, it's important to ask if artists are allowed to rent these spaces, as landlords might be concerned about paint stains and similar problems.