I don't know where to get a replacement battery? It shows SEL CB4-6, (6V 4AH/20HR) on the battery itself?

This is for the 5000 but it should be close.
5s2p 10 4Ah Li ion 26650 battery for the Bladefish 5000 underwater...

Try this first.
How to Fix Dead Lithium Ion Batteries That Won Hold Charge Anymore

I do not know what kind of scooter you own, but this is a charger on amazon. If you include more info I can check for compatibility. NUMAX Mobility Scooter Battery Charger 24V 4AH 4AMP Connect Forget

you could try the original supplier or ebay and failing that you could try searching online for a pdf manual and just buy any charger that is the correct type voltage and most importantly polarity, having looked at the bike its a childs harley type bike and is 6v 4ah and likely a sealed lead acid type so i would buy a 6v 500ma lead acid charger, if there is a charge port you will need a plug to match and you will need to check the polarity is correct or if there is easy access to the battery terminals you could just use the crocodile clips to the corresponding terminals on the battery itself there is a selection of chargers here 6V Lead Acid Battery Charger eBay a 500ma charger will charge the bike in 8hrs the higher the ah the faster it will charge but i wouldnt go much higher than 500ma without checking with the vendor that it is suitable for a 4ah battery hope this helps

replace main system battery. this will either be a 12volt/4ah battery or a 12volt/7ah battery.

you can order one here www.powerwheelsoutlet.com

I believe the replacement battery is a CA645 - 6V4.5AH/20HR. You can check your particular model number at: http://www.batterymart.com/p-6v-4_5ah-sealed-lead-acid-battery.html

You can order it from Black and Decker, Vector sold all their product lines to them in 2006 and they carry the replacement battery cheaper than anywhere else I found.
http://www.dewaltservicenet.com/Products/ProductDetails.aspx?pid=69262&tid=16032

Open the control box and the 12V 4AH battery is inside. Buty new battery connect black and red wires to black and red terminals on the battery

AlarmProfessor.net

Exact replacement (HYS660) with full 6AH capacity from
http://www.batteryinternational.com/vector-hys660-spotlight-batteries-vector-f1.html
at 9.95 each.

I just ordered 5 batteries of same size and spec from:
http://www.atlantalightbulbs.com/ecart/nw012104/YUNTONGYT-660(6V6AH.20HR.htm
$9.34 per plus shipping.

I found another compatable but it was only a 5 Amp/hr battery. The above link is a direct replacement for the SEL DMU 6-6 6V6AH battery that came with the dual spotlight.

Measurements, electrodes exactly the same..

They just call it a YT-660 made by Yutong...

You are welcome...