CBRS (3.55-3.7GHz) has been available for over a year now in the US. I’ve worked with a few mines that are deploying it to get a larger coverage area than WiFi. You should start by speaking with one of the SAS vendors, e.g. Federated Wireless.
You could try the Xilinx ZC706 with an ADI9361 based FMCOMMS board. The OpenWIFI team has a few configurations listed on their readme that are popular: https://github.com/open-sdr/openwifi. I think these setups will still cost >$1000USD and require considerable effort to get going - I don’t know of a <$1000 SDR setup for 5G development that would be easy to setup and get going with. Curious if anyone knows of one.
Plenty of options with varying levels of cost and/or complexity. For example, you could try Matlab’s HDL coder/Sysgen and an FPGA eval platform with eval board transceiver. National Instruments also has a good platform to start with using Labview and the Ettus USRP devices. OpenAirInterface has been developed to work with the USRP devices and has a fairly active community led from Eurecom.