Where can I buy the carrycot cover?
For any spare parts - even the bassinet leg rest cover, you can only contact the seller of the pram or contact the manufacturer, the Seka, directly. You can’t get these parts just so, anywhere. So I will recommend you doing just that.
You can also go to a stroller shop and ask if they have any universal canopy/sunshade that could work for the car seat.
It’s not a very quality/safe car seat, to be honest - it’s the cheapest Polish-made car seat one can get. So thinking about a different - ADAC tested car seat and the reasons why a safe car seat is better for your baby one might be more beneficial in this case.
Lastly, a thin footmuff or a blanket with elasticated sides can also be used to protect your baby. Again, a stroller shop around might help you with that.