There was National Sign Bid, but they have been gone for a while.
Some find and post work on signsearch.com
Some use the Dodge Reports but it can be pricey.
Jobs can be posted on The Sign Syndicate
A new resource that looks interesting is
http://sign.guide/index.php/en
where we can list our company and local contractors and designers can partner.
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What you are proposing is going to be a little difficult. Most of the work
you describe usually requires a contractors license so jobs need to be local.
Most shops using/needing leads need more than a monument here and
there, or some random interior sign... plus those small jobs can get
cheap when chump sign shops are competing on price.
Who's going to post a wrap job? Those shopping design to the cheapest
company most likely.
If I was an archtect/envrionmental/experential graphic designer, contractor
i probably have people I want bidding on a job... I also cringe at some sign shops
where they think a foam sign is the standard way of making a sign, or are
clueless about codes, ADA signs, heck, they don't even read plans... flatten
dimensional signs, cheap out on materials, cheap out on craftsmanship.
As a designer, if i'm going to hand over a 200k+ job, the shop better know what
they are doing.
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I'm lucky that in my area, work is posted on a few sites for free. Some recent, many
years away... were talking millons of dollars in signs and design, yeah we have to do
some footwork but once we establish a relationship we usually turn that into sizable
repeat work.
If I was a signshop that does more than vinyl, I would evaluate what I can make, sub
out, then hook up with an experential designer, contractor or architect using a design-bid
and/or design/build model