Where can I find a narrow booster seat (for occasional use only)?

[Snowstorm]

Got 2 'big' seats in the car for DD's 5 and 6 years old. Need to try and squeeze a narrow booster seat inbetween for when I bring the occasional friend home.

Wondered if anyone had already been through this because when I go on line they just refer to these small seats as a weight and age thing and not the actual size of the seat.

TIA.

tesco do a really narrow one for about £6

I second the tesco one, it was all I could fit between 2 car seats in my old car.

hmm it doesnt seem to be on the tesco website,maybe they have it in store?
its plain black,

Is it just the bottom cushion bit or does it have the two arm bits sticking out that you put the seatbelt under/across the lap of the child?

Sorry, rubbish description!

hmm not sure it was my sis-in-law who has them,maybe larsazlol will know,