If the dominant brand of PLCs in your area is Allen Bradley, the cheapest thing would be to buy Micro 820 PLCs and use the CCW software which has a free version.
CCW is closer to Siemens than the other Rockwell products...
There isn't a lot of detail in your request, so here's my thoughts:
- What budget do you have? This is key to hardware and hopefully some licenses on the cheap.
- Do you know which platforms are more common around you? This will direct where you'd spend money.
- Even if you have one platform used around you, do make an effort to teach other platforms. There's a benefit in doing so for your students since it opens the doors to move elsewhere or catch a break in a different industry.
- Do teach at least one SCADA platform (most likely ignition), nowadays just knowing PLCs doesn't cut it and the interaction with the HMI is interesting to develop your PLC logic too.
- If budget is indeed tight, get a cheap platform (Automation Direct or something like that) to teach the hardware part of a common PLC (how to wire stuff to it) and then focus on, for example, Codesys or Twincat to teach the basics of PLCs and PLC programming.
- Get in touch with automation vendors... if the most relevant platform around you is Rockwell, there's a very good chance Siemens will be interested in providing some stuff for you.
- Do get in touch with local companies too and ask whether they have obsolete, but working equipment they can give to you once a machine is decommissioned. You may have some trouble getting software licenses, but a PLC is a PLC and knowing how older versions worked is always a useful bit of knowledge (like, the first PLC I interacted with was through a console the size of a calculator).
Good luck.