As Roverhi said. You need to make sure its 316 stainless.
Depending on what country you are in, there are plenty of online retailers selling marine
hardware if your local hardware shop cant get what you need. But you could also try industrial hardware guys, rather than the handyman type of hardware shop.
Go with eye bolts and large washers.. Just remember to use plenty of goop (silicon/sika etc) when you bed
If you think your transom is weak and want to distribute the loads, you could use Pad Eyes instead. But that would be unlikely.