Reddit mentions of Tiny But Mighty: Kitten Lady's Guide to Saving the Most Vulnerable Felines

It may be that long for what a beginner would start with, which would be kittens at least at weaning age (around 4-5 weeks old) who are coordinated enough to really start playing and socializing. With or without their mom (sometimes they'll come in with a rescued mom who can be a stray or a feral, and sometimes they'll be found as orphans), past 4 weeks it's easy enough to teach them how to cat and to be well-adjusted furry members of their future forever families. Spay/neuter surgery has to wait until they're at minimum 8 weeks of age and 1 kg in weight (although some rescues and vets may prefer to wait a bit longer).

You'll probably have them a bit longer past their spay/neuter surgeries if it's the rescue organisation's (and your) preference that you be more involved in screening potential adopters and matching your furry little graduates with homes which would best suit their "purrsonalities" and reduce the risk that they'll just be returned to a shelter or otherwise abandoned in the future.

With kittens under weaning age, with or without a mom to do half of the work, you need to be especially prepared for complications which require hand-rearing, possibly tube-feeding, and giving meds and fluids (this is more advanced stuff which needs to be learned from a veterinary professional or an experienced rescuer). Sadly, it's normal in litters that they don't all always make it (outdoors, they average only 1 out of 4 reaching adulthood. Indoors, you can improve their odds greatly, which makes the occasional sad loss more bearable). Sometimes there'll be one born with congenital abnormalities which aren't always obvious by looking at them, sometimes they fail to thrive without anything appearing to be physically wrong with them. So I don't recommend getting any that are that young, to start.

My favourite rescue in the area is Tinykittens in Fort Langley, BC, who specialize in TNR of feral colonies and are internet-famous for their 24/7 kitten cams, Cassidy the Miracle Kitten, Grandpa Mason, and other viral rescue animal videos. They technically accept volunteer applications, but they're small and they take on some really tough cases, so they recommend contacting Langley Animal Protection Society (LAPS), whom they work with closely. There's also the big one in the Vancouver area (whom Tinykittens has also worked with on larger rescue operations), the Vancouver Orphan Kitten Rescue Association (VOKRA). Tinykittens is still a great source of information on ferals and tough cases with terrible odds, pregnant moms (feral or stray) and the realities of kitten birth (watching a cat give birth live is both gross and exciting. Guessing the number of kittens and their coat colours is kind of fun). I also love Kitten Lady Hannah Shaw as the expert on orphan neonates. She's not local, but I recommend her fun, informative, and instructional videos and other materials. She also has a book coming out in a week called "Tiny But Mighty" which is a comprehensive manual for doing what she does, really.

Even if you're not in the Greater Vancouver Area, you could probably contact these bigger rescue organisations to find out who is reputable wherever you are in BC, because they may have worked with some rescues from outside of the territory they usually cover. Like during past wildfire seasons, I know that animals needed evacuation from fire zones, whether they got separated from family and needed somewhere safe to stay until they could be reunited, or they were strays or ferals which the local shelters and other rescuers were trying to evacuate to safety. That's another fostering option to consider for this summer. July has been pretty mild for heat and wildfires, so I am seriously side-eyeing August, expecting it to be brutal. >_>

Anyway, sorry... I rambled. I'm passionate about animal rescue, and I'm also a little bit medicated and therefore talkative😅. I hope the info helps.