A friend and I are doing a similar party this coming October, only ours is a seance party set in the 1920s (these sorts of events were popular up through the 20s and early 30s). We picked the 1920s mostly because I have a 20's style bob haircut...
My friend is a magician and mentalist, and most of our effects have been purchased from commercial magic suppliers. Have you looked yet at any of those? There are thousands of online magic dealers. We like Hocus Pocus Magic in California, but really any good magic shop could set you up. Here's some links:
http://www.hocus-pocus.com, http://www.magicfact.com, http://www.daytonamagic.com. If you're in England, there are also lots of magic dealers there. An internet search should turn them up. When you get to a site, you'll find these sorts of effects usually listed under headings like "mentalism", "bizarre magic", or "spooky effects." Everyone does it different: you kind of just have to poke around.
There is a WONDERFUL book on this subject by the magician Eugene Burger called "Spirit Theater" (another name for this sort of event). He had a long-running act of this nature that he hosted out of a spooky old house in the Chicago area. This book has clear instructions for many of his effects, and great history and anecdotes about seances in general. It's not available from regular booksellers, only from magic dealers. There are many, many books on this subject that you can find by browsing magic shop booklists. Denny and Lee Magic Studio has a huge booklist. They're at:
http://www.dennymagic.com.
For our party, we're planning a floating table, some slate-reading, objects moving on their own, stuff like that. At the Hocus-Pocus site, there was a whole kit you could buy that I believe was called "Manifestations": it was a collection of audio tapes that taught routines. You can also buy little "spirit cabinets" from which all sorts of objects like trumpets, ectoplasm, and such things will fly out, as if they're being tossed by spirits. Hocus Pocus sells those too.
Good luck to you!
Tracy