From what I've seen, the bowl gasket starts off as a large o-ring. Take the bowl with you to NAPA and see if they can get you one(four). I say Napa because Autozone and Advance don't have a big selection of o rings, just assortment packs.
I believe I read a trick here that you use scotch tape to keep it in place and hold that shape while you're putting it together. Double check that though 'cause I haven't done that yet.
That's where I'd start. Maybe if you look in the fiche like on hondaparts-direct.com or others, it will give you the specs for the o-rings.
I'm new to this, so I hope it helps.