I have my own balloons for use, like to ask if anyone knows where can I go to to inflate them with helium and how much is it usually?

You can consider renting or purchasing disposable helium tanks. http://www.adamevestyle.com/helium_gas_rental_balloon_accessories This shop has both options. Rental is from $80/tank and the disposable one is selling at $55/tank. http://www.partywholesale.com.sg/product/balloons/accessories-en/helium-and-rental/ This website also has different options that you can look through. Hope this helps!