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HackRF One is an option. There are a number of others. Legality varies by country/frequency/power level.

Can also make a Raspberry Pi act as a transmitter (https://github.com/F5OEO/rpitx), which could hit the zwave frequency but not the zigbee one. (Also, make sure to put a low pass filter on it.)

Or even better, a band pass. The output from that pin is really dirty.

I wouldn't recommend it for anything that will actually transmit into an antenna. Even with filters.

Gnuradio

Yup, lots around. The hint is to look for 'full duplex' SDR.

Doing zigbee or zwave stuff is a lot easier with zigbee and zwave than with SDR. Just use homeassistant with something like zigbee2mqtt, and you can control your LIDL light with an Ikea light switch and xiaomi movement sensor without any issues.