@MRCUR is correct that you will (perhaps unfortunately) still need to communicate with the supplier you parted ways with, and they will need to unclaim the devices from their Dashboard Organization's inventory, so that you could then claim them. There is no way for you to simply take ownership of those APs, even though they may be yours. Unfortunately this is one of the reasons why there should always be at least one user or group of users in the actual customer/business who has Org admin rights. Meraki Support cannot just go pull "your" devices out of "someone else's" Organization and assign them to another, even though these are actually your APs.
Your (former) supplier would simply remove your devices from the current Dashboard network, go to their inventory page and unclaim them, and once unclaimed, then you can claim them into your own Dashboard organization. Ask them for the list of serial numbers as well, really simple for them to grab them off the inventory page before they unclaim. Then you claim them into your Org and open a Support case to see if/what can be done with the licensing. While it's possible you could retain some licensing via an Org split, it depends on when & how the licenses were applied, so Support would need to review that and advise on what can be done.
I'd open a Support case first before proceeding, and agree on the procedure/steps, so they can review the current Org and its licensing state, but there too, you might have to get the partner/provider to open the case from their Dashboard. If you don't even have a Meraki Dashboard login, call Meraki Support directly on how to best proceed. https://meraki.cisco.com/support/
