I've got 15 years experience in insurance, primarily in liability insurance for businesses.
You can't just buy an umbrella policy by itself, it needs to sit over primary policies (generally, a business owner's policy for small businesses, and maybe auto or workers comp/employers liability). It's the same reason you can't just buy a personal umbrella policy, you have to buy the homeowner's and auto policies first.
Business policies are pretty cheap, especially if you're not doing a lot of sales and don't have a lot of inventory. There are a ton of carriers trying to get into the "micro" space right now, so I'd recommend reaching out to an independent insurance agent to help you find one. An independent agent is one that works with many different carriers, so they can shop around for you, as opposed to agents like State Farm/Progressive/Allstate that only work with one carrier.
If you are going to be retailing supplies from someone else, you may want to enter into a vendors agreement with them that would provide you with additional insured status for liability from their product. This wouldn't help you with your own product liability (labelling, kit instructions, etc.) but is generally a good practice, and pretty common for retailers.
Whether or not you want to buy an umbrella policy over your businessowners policy is really a question of risk appetite. You'll have more coverage in case your product seriously hurts someone, but a lot of small businesses have decided that if they get sued for more than the standard $1,000,000 occurrence/$2,000,000 aggregate limits, they'll just close up shop. And the vast, vast majority of small businesses don't get sued. Insurance companies make a TON of money on liability insurance, and umbrella coverage is the most profitable, which means that as a policyholder it's less likely to be "worth it" for you. But maybe the peace of mind is? It's really up to you.
Feel free to DM me with any other questions.