Where can I obtain a battery cover/door for this model(red)

Try searching eBay for Canon battery door (and other) parts.

1) They are model-specific (as you probably know), but there are a lot listed there. There is constant turnover, so keep checking.

2) You can also list an ad for the part you want, and see if any seller responds with one for sale.

3) Your best bet might be to look for your model camera as a 'parts only' listing on eBay. If you find one, you can probably buy it and just take off the part(s) you need for replacement on your camera. I just saw an entire Powershot Elph 300 HS on eBay for $11 with free shipping. The battery door was complete and intact.

4) You can also call Canon and get the specific part number. It may make your search on eBay or Google easier.