"Now tell me, where can I get those from?"
It's spring in the Northern Hemisphere. Grow it.
The Wife and I kept ourselves in fresh greens in three five gallon buckets for a year or so. Spinach, Russian red kale, arugula ("rocket" to you Brits), and various lettuces work well and are good learner veggies. Select "cut and come again" varieties and harvest the larger, outer leaves as required, leaving the inner core and the roots behind. Park the eldest planting outside in sunlight if you can (an apartment balcony works), the second planting indoors in a sunny spot near a window, and the third freshly seeded bucket can sit pretty much anywhere until the seeds sprout, then place it next to the second bucket. Rotate the buckets about every two weeks. You can get around 9 plants in each bucket (note this is NOT head lettuce, you are growing it for the leaves).
If you have an un-used window box, separate it into 4 equal parts and succession plant that instead of using the buckets. (The three buckets above is a minimalist approach, 4 works better.)
If you have the space you can grow so-called "baby lettuce" in 10x20 nursery flats. You can fit four flats on a four foot shelf, under a light. No need for a proper grow light, a simple LED "shop light" works well for baby plants. Rotate weekly. You can put four shelves together using cheap roll-around wire shelving, giving 16 flats. Share with your neighbors. This method also works for microgreens (call it "food confetti" to get your kids to eat it, if you have problems getting them to eat veg).
Don't buy seed in little packets, get it as "seeds for sprouting" or "for microgreens". You'll save a ton of money over the long haul.
Note that this is a great project for a household with kids. Most kids will happily eat any veg they help grow.
As a word of warning: If you grow your own, you'll never want the near flavo(u)rless supermarket variety again. Don't say I didn't warn you.