Unless your railroad has a military theme, you will have a difficult time finding vehicles in 1/48. There are many military vehicles in 1/48 including trucks and some cars. But they look distinctly military so would need modifications to work for most layouts.
The choice is really between 1/43 and 1/50 scale vehicles which are popular scales for model car collectors. Being frustrated by the lack of 1/48 vehicles, long ago I decided to make the standard for my layout 1/50. Let me bore you with a quick comparison of the three scales:
In 1/43 scale, 1 real foot = 0.28 inches.
In 1/48 scale, 1 real foot = 0.25 inches.
In 1/50 scale, 1 real foot = 0.24 inches.
So the average 15 foot long car/truck would be:
4.20 inches long in 1/43
3.75 inches long in 1/48
3.60 inches long in 1/50
So the 1/43 model is almost 1/2 inch too long (4.20 - 3.75 = 0.45), while the 1/50 model is only 3/20 inch too short (3.75 - 3.60 = 0.15).
There are many nice 1/50 vehicles on the market (Corgi, First Gear, Lledo, Norscot, Ertl) and some no longer made that can be found on ebay and at swap meets (such as Athearn, Matchbox). On ebay there are dozens, if not hundreds, of 1/50 vehicles that get posted every day. As Jeff notes above, he uses a good strategy so that he doesn’t overpay. Patience is the key. Thousands of these models were manufactured so they come up for bid on ebay all the time. If you miss one because of price, it will most likely appear again within days or weeks.
I am currently working on collecting 1/50 vehicles for my layout that I am just stated to build. Here are a few photos of some of the 1/50 vehicles I have. These will eventually get modified and weathered when I put them on the layout.
