Hi Leslie, how unfortunate your MIL had that health problem and was unable to continue to keep him...
I think you are on the right track - contacting a breed specific rescue. You could google up "chihuahua rescue indiana" and see which others might pop up (contact several since so many are innundated with dogs now)... if you are unable to keep him at your house, do you think you could let your MIL keep him (since it was a stable situation for him there for 4 years) with you stopping in to walk him, etc ... the things she is now less able to do ... until a perfect forever home comes along? I'm just thinking it would be less bouncing around. Of course, I have no idea how far away she lives from you...
I think you are on the right track - contacting a breed specific rescue. You could google up "chihuahua rescue indiana" and see which others might pop up (contact several since so many are innundated with dogs now)... if you are unable to keep him at your house, do you think you could let your MIL keep him (since it was a stable situation for him there for 4 years) with you stopping in to walk him, etc ... the things she is now less able to do ... until a perfect forever home comes along? I'm just thinking it would be less bouncing around. Of course, I have no idea how far away she lives from you...