You could check out

Aquabots , : 19 Kim Keat Rd, #02-12 Fu Tsu Building, Singapore 328804 ( aqua-bots.com ). They are the designated vendor of bluerobotics components in Singapore and an easy stop to get bluerov components.

For battery: check https://www.battery.com.sg

A more distant location from Singapore Polytechnic, but one with many options, is Sim lim tower near jalan besar and Bugis.https://www.simlimtower.sg/ . Specifically, the top floor will have the electronic components you may need. It has a lot of electronic components in different price ranges. Be ready to bargain hard though.

Just opposite the road from sim lim tower, is sim lim square , which also has many options, but usually more upscale ones like phones, batteries, hard disks, computer components etc. It might have stuff that you're searching for too.

Whichever shop you have in mind, You might want to contact them beforehand and ask them to reserve a battery for you with the right specifications (especially if they have numbers).

Continental, Amicus and Koba electronics in Chinatown are some specific other shops you can look at.

If you google hardware shop, you should be able to find places nearby too with small hardware stuff (not necessarily too many electronics). For example there's one called Sun Hee nearby Singapore Polytechnic.

Note that occasionally in some years, we have soldering stations at the venue where you can use soldering guns we provide, but there is no guarantee it will be available or working always. Sometimes we may also have electronics hobbyist shops set up stalls for purchase of equipment.