Open an endangered monster reserve
Put the fear of god in them
In order to preserve an endangered species you'll have to endanger it on the first place. Choose a species from any almanac and force it into the brink of extinction. Go wild with your methods: poison their food sources, burn their lairs by the dozens, write their name on the wanted monsters section of the newspaper so folks will hunt them down for you.
When the population is severely decimated, round up the creatures and stuff them into your makeshift reserve. Make sure your hosts can find themselves comfortable in your reserve, else you may experience a great turmoil.
Building a reserve
For starters, you should check if there is any property on lease or even better, abandoned. Farms, mines or cottages are all good options for a would-be reserve. Pick one and start to patch the holes, and refurnish the floors.
Right after you should plan where the feeding area would be located; as well as the pens, the playground and your dwellings. The latter should be far away of all the former. Don't forget that this it's not your everyday zoologic. Your specimens are deadly beasts who are quite upset with your works.
Finally, stock yourself up with the supplies and bandages to survive for a while in the reserve. Bring on the poor monsters.
Tending the beasts
Make sure that they are well fed and well treated. Also see that they reproduce at a steady pace so they can soon be released again into the wild. Keep an eye on your own safety all the time, this creatures will take any chance to take revenge against you.
Soon enough an interested researcher will buy off your reserve, paying handsomely for the chance of stamping their name on the next breakthrough discovery on the field of monster science. As for you, you'll get a couple gold bags and hopefully something else.