If price is a concern, do a compare first... A roll of film can vary by £3-£4 per roll depending on where you go. Some films are cheaper on some sites - it really does depend which film you're looking for.
Also beware sites that quote prices ex Vat.
Analogue Wonderland (I tend to buy here mainly because of reward points)
Speed Graphic (can have good deals)
Nik & Trick
Wex
Silverprint
Process Supplies
Discount Films Direct
7DayShop
I'm sure others will chime in with more.
I avoid Amazon because they seem hugely inflated.
eBay can be good, but can also be hugely inflated.
Expired film is silly money for the unpredictability.