The insurance company I use for my instruments is American Collectors
Insurance: https://americancollectors.com/insurance/collectibles/ 

You do not need an appraisal, as they write "agreed value" policies.
With your application you send them photos and a short description of
the instrument along with the amount of insurance you want on that item.
If they agree, you have that much maximum coverage. Items under $2000
each have blanket coverage up to a set limit.

I have a rider that covers my organs even at rallies. The total cost
of coverage is about 8/10 of one percent annually, which is less than
a fine arts rider on my homeowners insurance (and they need appraisals).

Note that American Collectors Insurance primarily insures classic cars
on the same basis, so their website emphasizes that.

Mike Schoeppner
Kansas City, Missouri