Hi Melody
Welcome to Bali 
• postcard price varied from $ 0.5 - $1 (sometimes $2)
• buy them at the stationary shops / bookshops in a mall (Gramedia shop or Periplus has beautiful range). You can alao buy them at souvenier shops (big oleh-oleh shop)
• you have to go and buy stamps at post office, give them back to the cashier , do not put it in the mail box–they never check. Mailboxes are just decorations here.
Note: leave enough space for barcode stickers from the post office (aprox. 1cm x 4cm). Its a way to track yr mail within Indonesia.
• if you are in Ubud, many small stationary shops around (ATK shops), otherwise google for ATK shops (or mall) wherever you are (Jimbaram etc.)
Hope this info could help