Some insurance firms offer 'shop policies' which are specifically designed for small businesses. Some don't want
to handle commercial policies - fair enough.
Contact a few and they might send round a rep and with luck you get a helpful one.
You basically need cover for all your equipment and raw materials. If you own the premises then you can incorporate
buildings insurance. You also (and this is more important IMO) cover for 'Goods Held In Trust'. i.e. customers things you
have on the premises that you are working on. You have to work out a base figure for this depending on what scale of
operation you run. I you have particularly valuable work in for a short time, you should be able to ring the company to
bunk-up the cover for a short period. Obviously the value of what you hold in trust is going to vary, but you can't realistically
keep tweaking the policy ever time some thing goes out/comes in.
Public Liability in case you get sued by some twit who trips over you doormat. This is usually quite cheap, but the rep will
advise you you of any potential hazards. (New doormat?

) I've got £2 million PL cover.
There are other little 'perks' that can be handy. Cover for Goods in Transit, etc.
They may insist on a few improvements - news locks, fire extinguishers and a few things you never considered.
Cost? Again, depends on the scale of the operation.