Best Surface Go Sleeve For Now
If you are planning to buy an expensive Surface Go case when you have the money for a Type Cover or when more options are out, you will still want to protect your Surface Go without a major investment.
The best option is the Lacdo Shockproof Tablet Sleeve. This is under $10 on Amazon and it includes a shockproof lining that will do more to protect the Surface Go than most basic universal sleeves.
Inside dimensions are bigger than you need, but that will allow you to slip the Surface Go in and take it out easier and it's not much bigger than the Surface Go. This will also likely fit the Go with Type Cover attached if you need a sleeve for that purpose.