I have multiple Modbus masters that need to communicate with 1 or more Modbus serial (RTU) slave devices. What product should be used??

The best option (straightforward implementation, most flexible option) would be the PLX31-EIP-MBS4 or the PLX31-MBTCP-MBS4.

These gateways have four configurable serial ports that will allow data from 1 or more Modbus serial slave devices to be shared with multiple masters.