Most ISPs will offer an SLA of some nature. But the pricing for five nines (99.999% uptime - offline for 6 minutes a year) or four nines (99.99% uptime - offline for 52 minutes a year) and your brain will hurt at the price for compared to a standard home ISP. Even three nines (99.9% uptime, offline for 8 hours and 45 minutes or so a year, and considered woefully inadequate for most serious businesses) is painfully expensive. And then you get to the question of how do you define uptime? If you have a 20mbps connection - is it still up if you're only getting 10.4mbps download speed? What if you're only getting 2mbps? How are you going to measure that? Has your lawyer been through the contract? The contract from the vendor will start as a wishlist for them where sure, you have an SLA - but it will usually utterly unenforceable and you can never, ever collect on the penalties for it. The penalties will be trivial and in a dispute you'll probably have to pay the vendors legal fees. You'll need your own lawyer to go through the contract and negotiate it back to reasonable. A lot of ISPs will just tell you take a hike - the contract is the contract, no matter how unreasonable it is, unless you're dropping 6 figures a year.

If you do have an on hand lawyer who can negotiate an actually enforceable contract where you have a way to measure and track up time that you both agree on - you then have the hassle of measuring and tracking up time and keeping records for compliance enforcement. If you actually try to enforce your SLA you're probably going to have to get lawyers involved again which gets pricey quickly. Did you include legal fee cover in your penalty schedule? And what penalty schedule are you going to ask for? Because it needs to be something that the business selling you a service is prepared to risk - sometimes you breach SLAs - unavoidable fact. So you need to charge your customers quite a bit to insure yourself against those costs. But collecting on an SLA breech is a bloody hassle so you want to make sure that the penalty is substantial enough to justify it.

TL;DR - there are a bunch of boring, complicated and expensive reasons why nobody does SLA agreements for home users. But mostly it's expensive and difficult.