Get them into swimming lessons! Although you probably need another pair of hands if you're gonna do them both at the same time....
Infant lifejackets usually have quite big collars for supporting their heads for floating and will generally have more padding in the front (to make the jacket float the kiddy on their backs) so can be a bit tricky trying to get them learning to swim with a jacket on and you kinda have to counter balance the jacket to keep them upright and comfortable. If you do use a lifejacket make sure it's good quality and has a crotch strap so they can't slip out of the bottom. (This is from my experience as a swimming teacher who has used lifejackets in survival swimming classes....)
Unless you're into sailing/watersports maybe just try arm bands?
As for wetsuits I have seen them around Orchard but can't think where exactly off the top of my head. But don't pay through the nose as they'll keep growing out of them
