This is a fairly trustworthy site to buy paving slabs and other construction materials. It checks out on on all sources I usually use: domain registration, physical address, reputation sites and a quick search for complains. So go ahead and look around, but remember that for larger purchases you should consult an expert. They also have a fairly standard return policy, so if you do find out that you don't need a certain item, you might still be able to salvage some money.
External pavement is tricky in general, so you really need to consult somebody who done it before. It requires some kind of foundation (mostly compacted sand) and manual work to lay each piece. When finished it can look very nice, but it can also be very adversarial to people / children who happen to fall on them. I would recommend grass, asphalt (possibly a small strip of it for the tires if we are talking about a garage) and only after that paving with slabs. Good luck with the construction!
Full disclosure: this is a paid review from ReviewMe. Under the terms of the understanding I was not obligated to skew my viewpoint in any way (ie. only post positive facts).