> i need to have those items valued by an external company
That's usually how it has to be.
If they know what it is, they know what it is worth (and they can probably get it less than you paid).
If they do not know *exactly* what it is, and it is destroyed or stolen, how can they know what to pay-off?
Old cars face the same problem. The "1939 Hudson" in my neighbor's garage may be worth $400 (scrap value of a rusty corpse) or $40,000 (concours quality restoration). Pictures would help, but don't tell the whole story; and owner-supplied pictures could be of a different (better) car. There's guys you call who come look at the car, know the market, document the condition and serial numbers, set an appraised value, and are nominally professional (won't take small bribes for high appraisals).
In cars there is also "Agreed Value". You pick a value. If the company agrees the number is not absurd, they will offer a rate based on that value. Actually a higher rate than if the car were Appraised: you could high-ball the value, pay for a year, then have your buddy steal the car. Same as homeowner insurance usually won't be written for much more than the house/land is worth (in the company's opinion)-- it tempts you to have a loss and recover more than was lost. (This made insuring my 1837 house interesting: boulders and logs are not modern materials, they don't know the cost to-rebuild, it could not be re-built as-new, even the foundation which would not burn could not be rebuilt upon under modern Code-- we finally agreed on a same-size new-stick house plus minimal new foundation.)
So: you could try to get an "Agreed Value" that your 2003 Sam Ash Student Special is worth $9,000. Since the insurer has some idea what a Sam Ash is, they may decline. If they accept, they may want nearly $9,000 for the first year, because $9,000 coverage on a $90 axe is really begging for fire/flood/theft ASAP. (Maybe you are honest but insurers see the worst in people.)
Any large town has a used-guitar shop and the guys in there can ballpark the value of almost any guitar. You will have to pay for this... not just for their time, but because their main racket is low-pricing guitars they buy; they need strong motivation before saying a fair price. Exotic guitars in a small town, you should find a larger city.