When my husband was stung by a scorpion we went to Cruz Roja. Dr. Sam told us they had the anti-venom shot but it was very expensive unless you had Seguro Popular in which case it was free. We said yes, we have Seguro Popular and we have a copy of our SP registration in the car. Then Dr. Sam said they would only give it if my husband was showing an allergic reaction, which he wasn't. They observed him for an hour and then sent him home.
Second scorpion sting -- in the middle of the night -- was very bad. One of those little brown scorpions, and it stung my husband's finger 2 or 3 times, the pain was very bad. We called our doctor (Dr. Juan Pablo Loza), who was out of town and sent us to the Ajijic Clinic, who didn't have the anti-venom shot. Doctor recommended a 24 hour clinic in Chapala but we couldn't find it. The next morning Dr. Loza returned early from his trip to treat my husband, gave him the anti-venom shot (which had an 840 peso price tag on the box), and then sent him to that Chapala clinic for 5 bags of a saline+pain killer drip. The shot helped the pain a lot, the drip bags helped a little.
To find that 24 hour clinic in Chapala: go north up Madero to the bus station and turn right on to Miguel Martinez. The clinic is 2 or 3 blocks down, on the right hand side of the street, in a 2 story white building. Very small little door with the word "clinic" painted on or close to the door. Sorry can't remember better instructions.
Miguel Martinez # 530