The only one I can think of which is very close to the A14 is Birds Tackle and Airguns, Gipping Road, Great Blakenham, Nr Ipswich, IP6 0JB, 01473 830683.
They're just off the A14 at Claydon just before Ipswich. I've only popped in there once to grab some pellets whilst passing through but they seem to have a good stock of all things airgun.
There's also Simpsons of Newmarket but they're a few miles off the A14.
Eric.