i just grew my 2nd tray of wheatgrass. if you google 'wheatgrass kits' and select 'pages from the uk' there are plenty of suppliers. i got mine from: http://www.browfarmwheatproducts.co.uk
and i found the price the cheapest.
the first time i ordered i just bought enough seeds/compost for 1 tray so i could try it and see, second time i bought from the above website enough for several trays.
it came with full instructions and having never really grown anything before, i managed it without too much hassle.
ive heard otherwise about powder vs fresh though, i heard that the right powder (grown in the right conditions) is beter than the fresh wheatgrass juice as it contains the whole plant not just the juice. im being vague here and i appologise as i cant rememember where i read it.