Beware of insoles that will make your feet sweat, or ones that will cause the top of your foot to rub against the "roof" of your boot (i.e. if there‘s too much arch support - recently, I had some orthotic supports made for me, and the podiatrist was more concerned about keeping them short enough in height ... but in the end, they were useless since they didn‘t have enough height to be effective ... sigh ...).
Spenco are indeed issued by the MIR, but ... remember - we usually get kit from the "lowest bidder" ...
Once upon a time I had shinsplints, and an Airborne buddy clued me in about re-soling combat boots with Vibram - now our wet weather boots have Vibram soles, and if the Army gives you orthotics they‘ll also pay for you to get Vibram soles on your ordinary cbt boots (it only took a little over 20 years for the Army to catch up ...)
You can also get Vibram insoles, but the best advice has already been given - take your boots with you to the store, and get something comfortable.
Also - you‘ll notice we get two pairs of insoles with our wet weather boots - that‘s so you can alternate them, and dry them out.
If you buy insoles, make sure you remove them each night - if your boots ever get too wet, you can take paper towels and ball them up into the toes to remove some of the moisture, but then remember to take the paper towels out before you fall asleep so that air can get into your boots overnight.
Good luck.