Try plaza singapura. they had some specialty envelopes like petal enclosures when I checked this week.
I have some contacts that can help. You can search online there are tons in the US but I haven't found a lot in Singapore aside from Art Friends. I think they sell it for $1-2 a piece. I bought some last month from their Bras Basa branch.
PM me the quantity and budget and I'll see if my contacts can supply you.
