If you want a new one, Halfords do them, otherwise always some on ebay. Erde & Daxara probabley best known names. All will have standard hitches & normal 7pin plugs. If your car has a 13pin you can buy an adaptor for a few quid.
Avoid buying a secondhand non branded or homemade trailer as this could have insurance implications. Your car insurance will cover you for third party risks while towing so there is no need for separate insurance on a trailer worth a couple of £100s if you don't want to.
I use an Erde 122 - the halfords type but bought it on ebay. IK use it with side extensions and get about 250kg of logs in it which is well within it's plated limit. Very happy with it.
Avoid the non branded home made type. Not only insurance issues but may also invalidate breakdown cover.
The free ad papers are quite good as well, but as per previous advice on here stick to branded names. We got a 6 x 4 branded with spare, removable tailboard for £100
------------- Just remember, if it is older than yours, smaller than yours but next to yours, there is probably no credit owing on it...
We picked up a good 4x3 from ebay 33 quid put a 2x4 post in each corner and made it up to 2' 6" high with board all told (already had wood) 33 quid and a bit of time and my free labour. Now have a perfect all uses trailer.
i used to have a duuo, my parents still have it and it has stood up to seriously heavy usage, carrying logs up and down to their fairly remote welsh cottage, moving furniture, trips to the tip etc, also very easy to store. I bought mine second hand from a local trailer dealership - you can get them off ebay too
Ours came direct fromt he owner whohad bought it from new, still had remenants of the last load they shifted in it (sand and gravel), could see the sand and gravel they had in the garden. Was well chuffed with it!
------------- Just remember, if it is older than yours, smaller than yours but next to yours, there is probably no credit owing on it...