Transair are good and who I normally use, but look on ebay first for most things as there are some real bargains to be had. For examle, In my case: Brand new headset, (for pax use not for me not DC) incl case £40 (Cost £120 in T/Air, AFE etc), Sportys E6B calculator £25 all in (Equiv calc cost >£60).
Sometimes you get a job lot of books, knee board, headset, ruler etc. being sold by someone whose giving up flying or hasn't started in the first place, these can be really good value for money. If you don't want all the job lot, keep what you want, sell the rest on.
Also check Amazon for books, my Trev Thom book 5 (Instrument/radio nav) was only £17 post free as opposed to about £23-25 elsewhere. You can get books off of ebay but be careful they are not too old and therefore a bit out of date.
Overall, as someone else said, don't go too mad buying stuff just yet.