I'm sure they maybe different ways but the most common is to add flaxseed to their feed. Of course unless it is part of your marketing scheme to sell you could also just eat flax yourself. The foraging is usually green plant material (and bugs) that give them a higher beta carotene content and that beautiful orange yolk.
Love to hear from some of our more nutritional experts. Hopefully they'll chime in.
Free ranging works here but i believe pendned birds benefit from ground flax seed and you can get small jars at the health food stores ,not sure where ya could get large amounts mabe the Internet can help on that