I've used both inkjet and laser decal paper. I do prefer using the laser paper over inkjet. But then I mostly print using black ink making numberboards or reporting marks and numbers. I seal all my decals with either gloss coat, or Microscale's Liquid Decal Film. I do have a little trouble with the inkjet decals wanting to run if I cut too close to the printing. Also, the inks used in inkjet(at least on my el cheapo printer) are designed for white paper from what I've been told and have read. I did try that and the colors do come out rather transparent with the lighter colors the biggest offenders.
I do not own a laser printer, but one of the big box office supply stores near me will print off a sheet(they won't use a partial sheet) for me from my flash drive. My next trip is for trying some color decals on laser so I've no experience on how well this works yet. The cost is less that a dollar. You could call around to be sure one in your area would do this as I've heard it's hit or miss.
What font are you wishing to use for numbers. Microscale, Woodland Scenics, and others do make alpha/numeric sets in several fonts with 4 or 5 sizes on a sheet of paper. Either water slide or dry transfer. Of course if you have several loco's to do printing your own would be more economical as your likely to only get 3-4 unique 4 digit road numbers from a sheet. Check their websites for examples.
