I see you are from the UK, so you will probably be horrified as I was to see very few children restrained in seats. Usually they are jumping or standing in the cars. Although you can buy car seats(we have the UK store Mothercare), I am not familiar with places to rent them-- considering the demand is fairly low. (there are no car seat laws here).
If you have a baby that is young and under 9 kilos, I have an extra car seat you can have. Send me a private message...
Probably your best bet is to post a wanted add on QL for these items, and you could probably buy them off someone for cheap. When people leave here they aren't too interested in lugging old baby stuff back home once their child no longer needs them.(I got my other car seat and a swing for 100QR total!- and they are Graco) Or, post a message on QL with the title "need to borrow car seat for XX kilo child and a high chair" or something like that. I'm sure some mom or dad who isn't ready to part with their stuff might be willing to loan it temporarily.
Lots of babies and kids here, I'm sure you can find stuff to borrow...