When looking for a replacement canopy for umbrellas, most important is to check that the new canopy will fit your current umbrella. Confirm that the number of rib pockets in the replacement canopy matches the number of ribs on your umbrella. And the size (e.g. 9 foot, 11 ft, etc…) is also critical.
Some replacement patio umbrella canopies can be found at these links below at a reasonable price and with a lot of positive reviews:
9ft Market Umbrella Canopy 8 Ribs Taupe (Canopy Only)
9ft Replacement Canopy 8 ribs in Hunter Green (Canopy only)
Of all the patio umbrella parts, a replacement market umbrella canopy is probably the one most often replaced. This is in part because the canopy is directly exposed to the sun and precipitation.
Umbrella canopy replacements can sometimes cost more that just getting a whole new umbrella, especially if shipping is free. So it could be better just to reuse the current base / stand you have, and get a whole new umbrella assembly all in one.