For mail order you might try RS Components. Their trade counters are strictly account-holding trade only but their website will allow you to order on a credit/debit card without an account.
Many localities will also have their own local suppliers. In Southampton, I've always used All Power Transmission in Totton and had no problems. In Poole there's Ashley Power. I don't know of any closer to you though.
Thanks Tony, SIP would not supply me direct a couple of years ago when wanting blades and used a perform blade.
Torbay not having any machinery suppliers I hoped someone might know if these belts are much alike perhaps the perform belt would do the trick.
Model SIP 01304 P/T.