A lot of sites do them of varying frequency.
Driver Wood does a boot sale every 4th Sunday of the month
Skirmish Airsoft Billericay partnered with Essex Airsoft Boot Sale for today's event. The latter had its own in Colchester back in February.
RIFT Airsoft's annual "Shift Your RIFTs" has a weekend-long boot sale along with a few retailers under gazebos in addition to the skirmish events
Red Alert has a boot sale every month or so
The Midlands Airsoft Fair is set to return to Newark Showground in June. Last year it had both boot sellers and retailers, the latter of which had a few freebies such as morale patches, small carabiners, etc.