We have device that works with 10 Mb. Some of the customers have 100 Mb, so our device causes problems. To change our device to support 100 Mb is going to take time and cost a lot, so is there any adapter from 100 Mb (or higher) to 10 Mb?
It rather depends on what exactly the "problems" are.
If the problem is that the customers port doesn't support 10 Mbps (uncommon right now but may well get more common in the future) then a simple Ethernet switch will do the trick.
On the other hand if the problem is that there is too much broadcast (or effectively broadcast) traffic for a 10Mbps device to cope then inserting another switch isn't going to help. You will need something that can allow the legitimat traffic to your device through while filtering out the unwanted broadcasts. Often a "home router" works well in this role with it's "lan side" facting your device and it's "wan side" facing the noisy network.
No such device exists, as to my knowledge. To be honest auto negotiation should work fine. It was standardised as part of fast Ethernet but is backward compatible with 10base-T. If auto negotiation doesn't work why not setup port where device is plugged to 10Mbps.
Otherwise you can use 10/100 switch between both devices.