This section is not specific to Essex maps, but rather applies to all maps.
Listed below are dozens of map dealers, mainly of antique maps, but also some who specialise in Ordnance Survey maps. I have no affiliation or link with any of these, and I am not endorsing them in any way, but I can say I have used several of them and have always found them very helpful and fast to respond.
After map dealers I have listed auction houses that hold regular auctions that have material numbers of maps and/or atlases.
Albion Prints have a great deal of prints and some maps.
Altea Gallery A major london gallery, although their website bunches counties together in random order that makes it harder to browse. They have a dealer shop in London.
Antiqua Print Gallery is mostly for prints, but some maps. They have premises in London.
Antique Map Shop Ltd A website-only dealer, although not many maps of Essex.
Antique maps & Prints A major website-only dealer.
Antique Maps Online.co.uk A large website-only dealer, although not many maps of Essex.
Antique Print Club An Australian website, so not too many English maps.
Antique Maps.com A very major website-only dealer, with a very large stock and lots of additional information.
Antique-Prints-Maps A major website-only dealer, with a very large stock.
Barry Lawrence Ruderman Located in California this is a good site for very old international maps, but not great for English counties.
Bryars & Bryars A West End dealer (close by Storeys Ltd) with a large stock of county (& other) maps.
CA Burden An excellent dealer for maps, but is the absolute top dealer if you want whole atlases - the stock of atlases (& maps) is huge.
Copperplate A major website-only dealer, with a very large stock (especially for Cambridgeshire and adjacent counties).
Crouch Rare Books A high-end West End dealer, specialising in the more expensive items - maps and atlases.
Furneux Antique Maps A major website-only dealer, with a very large stock.
Garwood & Voigt A major dealer based in Sevenoaks Kent, with a very large stock.
Gillmark Gallery has a huge stock of Hertfordshire maps in particular, plus all other counties, etc. Sadly it has moved from Hertford to Yorkshire, but still provides an excellent website.
Kevin Ryan A web-based dealer, who as well as having an internet business creates half-hour TV podcasts available on his site.
Kings Antiquarian The website for print and map sales of a dealer in Lyndhurst.
Kings Court Gallery A very large shop at the Putney end of the Fulham Road in London, with a very large stock of maps.
Kitty Liebreich A website-only dealer, and a finder of items for clients.
Laurence Oxley A dealer in Hampshire, based in a shop selling many things including maps. Does not have pictures of the maps on the website.
Lee Jackson A website-only dealer based in London.
Lesley Aitchison A dealer in Bristol who specialises in local history, particularly in Acts of Parliament. They have some maps and many other interesting items.
Lindisfarne Prints A website-only dealer with no stated address, but a Shrewsbury phone number.
London Map Gallery A website-only dealer accessed via eBay.
The Map House A very large West End dealer, with a large stock of maps, but no prices shown
Map Mogul A website-only dealer based in London.
Maps Perhaps A website-only dealer based in Tetsworth (Oxfordshire).
Mostly Maps A dealer with a shop in Hay-on-Wye, with a good stock of county (& other) maps on the website.
Old Maps.com Not a dealer, but rather a company that provides a voluminous list of map prices as a subscription on-line service.
The Old Map Man A website-only dealer with a good stock of county (& other) maps.
Paralos An Athens based dealer who is obviously strong on Greek maps, plus some from around the world.
Past Pages A website-only dealer from Kent that has a large stock of county (& other) maps.
Philip Sharpe Gallery A web-based dealer who exhibits at several fairs.
Prints Old & Rare An American website-only dealer, that says they only put a "small selection" of their "huge inventory" on the website, so you may not find anything.
Robert Frew A West End dealer with a large stock.
Roderick M Barron is a map dealer, but specialising in imaginary, comic, satirical, propogandist and pictorial cartography.
Sanders of Oxford An Oxford dealer with a large stock of maps and prints.
Simon Hunter (Antiquemaps.org.uk) A website-only dealer that has a large stock of county (& other) maps.
Steve Bartrick A website-only dealer that has a large stock of county (& other) maps.
Storeys Ltd A West End dealer (close by Bryars & Bryars) with a large stock of county (& other) maps.
Tooley Adams A website-only dealer that has a large stock of county (& other) maps.
Warwick Leadley A London dealer based in Greenwich.
Welland Antique maps A website-only dealer that has a large stock of county (& other) maps.
Dominic Winter is a specialist site selling maps and atlases at auctions 6 or more times a year; this is the major auction house for maps in the UK.
Old World Auctions is a specialist site selling maps and atlases at auctions 3 or 4 times a year.
Paulus Swaen is a specialist site selling maps and atlases at auctions 3 or 4 times a year.