... but most import mfrs. sell them as options. The term dealers here use is "boot." Floridians don't see them very often, I would guess, but they are fairly common in places like Boston, NYC and Chicago, where people do walk in all weather, and where it gets cold enough that a simple blanket isn't enough. You can buy generic versions that are made to fit almost any umbrella stroller.
I have to say that I think that the padding on the upper-end Maclarens is very decent. Not on the Volo, of course, which is mesh, or the next model up, but once you go better than that, it's about the same as what is on the seat and back surfaces of the Pliko. (Their old top-of-the-line, the Opus, was very heavily padded, BTW, but it was heavier, so it wasn't very popular. You might be able to find a used one.) You can always put a fleece underneath the child if you like; that's a much more compact solution, and it will protect your upholstery from sunscreen. Also, for a really small child, very soft, fluffy padding is not recommended on any surface where they sleep, as it is a suffocation hazard.
Something else to think about is that really well-engineered strollers don't NEED really thick padding to make the ride comfortable; the suspension takes care of that aspect. Here's a test you can do to determine a bumpy ride: go to the toy section of the baby store and pick up a loud rattle that sounds with very slight movement, then take it with you when you test strollers. (Toy maracas are very effective for this.) Put the rattle in the seat of the stroller when you test it; the bumpier the ride, the more noise the rattle will make.
BTW, getting back to the original topic, fitting the thing in a vehicle. Most stores will let you test putting the folded display-model stroller in your car; just go to the service desk and hand them your driver's license and a credit card as security. Of course, you should carry it to the car rather than rolling it, so as not to soil the wheels in the parking lot.