If you are placing signs on taxiways' - aprons , an alley etc then unless they are already in the custom poly section then its completely correct what Don says, you have to create your own. Then you will need to place one version in the TEXTURES folder in ADE this will be DXT version so that flight sim can see it and then in Textures_Dpy you place a BMP version - These will be the same graphic just one converted using DTXBMP.
By creating your own (A) you learn a great set of skills (B) you can make changes at will (C) you can add in the little bits of candy that are frequently missing.
here's some of Heathrow (EGLL) Im knocking together after becoming frustrated with online ATC bending my exhaust pipe for wrong taxi routes and not stopping in accordance with NATS charts (Not of course realising that old fsx scenery is nowhere near it and newer versions often don't get the taxi signs correct for hold points , closed taxiways etc.
These are the first rushes of the ground markers and some of the taxi signs being set up. If you don't want to go down the road of creating ground making you can utilise taxi signage as some APs actually have taxi signs that are full of information for pilot use; not just pointers to taxiways and gates



Here Im adding both HOLD POSITION lights because of the issue of washout when some aircraft lighting is used and a stronger ground paint to for the same reason. These can be tweaked according to test results now by simply making small edits to the graphic, overwriting it in the texture folders and then making sure that the Ground poly Texture is corrected again.
Yet to add are the gate numbers which I have created 3 sheets, 001 to 999) so that I'm not messing about with single digits and getting them located and the ground arrows which are more frequently used now than ever and can aid pilot navigation when on the ground -
By taking some time to learn the creation and editing of ground markers this will pay dividends later - Using the Custom Poly tool in ADE takes a little trial and error . Don't start learning it with the intention of "Ill get it perfect immediately" That won't work. Try a simple double yellow line near a gate or near a runway so you can get to it and look at it. Use a notepad if necessary to write down your steps of creation and placement - amend if required - look on you tube and read the manual as well.