A pet ladybug needs a diet of aphids, and a lot of them. The various types of aphids are not exactly endangered species, and most gardens and parks are full of them. Collecting as many aphids as your pet could possibly eat should not be a problem, and no gardener is going to object to you removing aphids. Just check that the gardener has not used pesticides on the aphids in the past few weeks so you do not poison your ladybug.
Find plants with aphid infestations. Particularly prone to aphid attacks are roses, fruit trees and soft vegetables such as lettuce. You could also ask the gardener.
Examine the plant for aphids. They often congregate on the stems and the undersides of leaves.
Hold a plastic tub under a leaf or next to the stem and brush aphids into it. Alternatively, tap the leaf hard so aphids fall into the tub.
Shake out the aphids into your ladybug's house. Tap hard on the bottom of the tub to dislodge them. Use the paintbrush if necessary but try not to squash the aphids. Ladybugs prefer live food. Also, too many dead aphids could decay and pollute the house.