Try reclamation yards they sometimes have ex council slabs cheap. You could also try asking on your local 'Free cycle' to see if anyone wants to get rid of some. Friday add may have some on offer? But for the amount you want plain concrete slabs would not cost a lot at your local DIY or garden centre.

Agree with Drc - put a wee advert in your local supermarket it they have a board - there's always folk wanting rid of them (either free or cheap) - good luck

Our local tip often has some. where abouts in Hampshire are you?

basingstoke

I got mine from wyevale they were about 3 pound each and i got mine at various stages to spread the cost. Ive got about 12 in my greenhouse.
If you have a look in my pictures you can see what type ive got Charlotte