I don't know if it's possible, but maybe you can measure the carseats you have now, and do a little searching online for different brands, and check what their width measurements are? That way you can see if the carseats/booster seats are bigger/smaller than yours, and make somewhat of a judgement from that?

That way when you actually get a chance to go to the shops, you already have an idea of what brands or particular seats are suitable/not suitable without having to leave home

Hope I helped!