Compared to the rest of the Metropolitan Los Angeles market, the Montclair region has lower rates, so you should have no trouble finding a great deal in this discounted region.
In the last 90 days, 0 listings have come on the market, which means new availabilities most likley are leasing quickly. You'll want to be prepared to tour spaces as quick as possible. About NaN% of the listings in the Montclair are subleases, so you will probably need to sign a longer term lease with the landlord directly. There aren't many listings below 2,000 square feet posted, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,030 sqft. Most buildings in this region were built in 1980 and are Class B.