I recently faced the same problem as well.

I'm not sure if this is the best method, but I was able to obtain libMathNetNumericsOpenBLAS.dll using the following steps.

1. Obtain the MathNET Numerics source code from GitHub (https://github.com/mathnet/mathnet-numerics).
2. Open "MathNet.Numerics.OpenBLAS.sln" in Visual Studio.
3. Build the "OpenBLAS" project within the solution.

The build requires some header files and import libraries, but they are included in the OpenBLAS release. (https://github.com/OpenMathLib/OpenBLAS/releases)