I wouldn't buy lady beetles. Instead, think in terms of a long-term strategy. I tolerate quite a few aphids on my brassicas early in the season because I see them as food to attract the beneficial insects I want in my garden. Beneficials include ladybugs, bees of all kinds, lacewings, syrphid flies, tiny beneficial wasps, etc. I've already seen ladybug larvae on my arugula and kale plants.

I have a year-round gardening climate, so I have something in bloom at all times to provide food and cover for overwintering beneficials. Plants with tiny flowers are especially attractive to beneficial insects. I grow yarrow, globe gilia, and California fuchsia in my veg garden, for instance, and I let carrot-family plants (parsley, celery, lovage) produce flowers and go to seed. If I had more space, I'd also plant native flowering shrubs with tiny flowers such as ceanothus.