1) will the 2-part resin putty stick onto the item after casting?
-> No won't stick, worse case give the mould a brush and wash in cold water. The resin putty will come off easily as Oyumaru is elastic.
2) where to buy the item?
-> Depends on where you are. you should be able to find it in a lots of online art and craft shop. Try google oyumaru
3) are they expensive?
-> Very Very cheap (Cost me around $6 AU a box, 7 pieces in each box)