The graphic novels are widely available from specialist retailers and bookshops can order them. Expect to pay around £12 per volume. 

The Manga comics are available from most comic shops for around £3 per issue.

Only the OAVs (known as 'Slayers: Dragon Slave' and 'Slayers: Explosion Array') and Slayers The Motion Picture are available here and only on VHS - expect to pay around £13 each.  Slayers: Dragon Slave is now very hard to get hold of but The Motion Picture is available from most specialist retailers and some high street shops.  It's always a good idea to check second hand video shops too.
 
The American DVD series box sets cost around £80 - £90 and the OAVs and Film are between £17 - £22 on DVD.  Most import DVD retailers sell them.

Action figures are £15, posters are around £6 and t-shirts £16.  Other items, such as wall scrolls and keyrings are not that common and the prices vary between retailers.