While you can easily make a bio-dome from plastic bottles for starting seeds and you can even buy a bio-dome from sources like Park's Seeds and seed growing kits from Burpee, there are a lot of options you can explore. You can make your own bio-dome from any plastic container that has a cover. Recently I found myself in a hardware store where these plastic bins were on sale and purchased one to use as a seed starting bio-dome.
I find that these kinds of grow kits marketed to newbie gardeners probably help boost their gardening confidence early on. Many may not understand that they could sow seeds directly in the ground or winter sow. If you don't know that a seed starter bio-dome isn't a necessity for starting plants from seed, you probably also don't know that you can use many items you already have around the house instead of buying a grow kit.
The humidity levels inside a bio-dome help the seed sprout in a couple of different ways. For the most part, seed need warm temperatures to sprout. Humid air is warmer than dry are, the humidity inside a closed container like this helps the seed germinate by making the air inside slightly warmer than the air outside the box. The humidity also keeps the seeds and seedling mix from drying out which contributes to the germination rates of the seeds and ultimately to the feelings of accomplishment of the gardener.
I used this plastic bin as a seed starting bio-dome to start seeds for lettuce "Garden Babies," basil "Cameo," and marigolds "Yellow Splash" that were given to me for free by Renee's Garden to trial and write about as part of a communal seed starting project.
One benefit of a homemade seed starting bio-dome from one of these plastic bins is that it is a lot more durable that the bio-domes and grow kits available from gardening companies and seed suppliers. It will last a couple of years and when not in use starting seeds I'll use it as a seed keeper for my seed collection.While the bio-domes from Park Seed and Burpee are perfectly fine products; If you feel like you must start seeds inside a bio-dome consider making one from household items to save you a few dollars that you can spend on something else in the garden.