From a trade supplier point of view, surely the place where you buy your stuff is the place to start. If they won't help out a bit, then consider shifting your business to somewhere that will.
As another said, what other string do you have to your bow ? If none, then get pally with others in complementary trades such as tilers, brickies, painters, window fitters, plumbers, kitchen fitters etc. That way you can advertise from small jobs to larger ones, though I caution not running before you can walk as any bad work from others will reflect upon you directly.
Think outside the box a bit. Who needs a plasterer and when ? For example, what about insurance work ? lettings agents as you mentioned, etc. I recently converted a cellar and though enjoyed doing the work myself as a hobby, I realised my skills did not extend to plastering at any level of professionalism. Had my building merchant been able to recommend one and (hugely important this) if I trusted their recommendation, I would have given one a go.
I would shy away from the Trusted Trader and Check a Trade lark unless you really think you can leverage it to your advantage. When I reviewed them, they appear very biased (you can delete poor reviews) and are simply a money making exercise. See if your council has a cheap / free alternative.
Also, what about offering a little work for a charity, school, etc. Word of mouth soon gets around.
Lastly, get your van wrapped or at least some signage. It has to be the best advertising you can get. Even park it (and pay) in the town / superstore car park at the weekend !
