I am sorry for your losses. That is unfortunately a risk we take when we take in buns with unknown or sketchy backgrounds. I would recommend going to a shelter and getting one that has been in foster care, or at the least vet checked and cleared as healthy. But, don’t worry, they aren’t as delicate as your experiences are leading them to seem!
I adopted Powder around three years ago and he was from a shelter, they thought he was around four when we got him but could be younger or older, no one knows his background. He’s still a healthy, happy bun though and runs around doing binkies/zoomies every day. Stickers came from a neglectful home (parents letting a kid be sole care giver) and she’s skittish, but the only health issue we’ve had with her for the also around three years we’ve had her is she developed a URI one time. So, it’s possible to get buns that will be healthy! Just have to be careful where you get them from. Stickers came from foster care and Powder came from the humane society, both got checkups when they came home as well just to be on the safe side. A healthy animal checkup when I get a new animal is a must for me.
I hope you find that perfect bun for your family. Sorry you have been through losing two in such a short time.