It is quite unfortunately that these bulbs got caught up in the Energy Bill passed in 2005. These 50 watt BR19 bulbs were unique as they were an incandescent that were also a spot light bulb. There are not very many incandescents that are manufactured to be a spot light.
So, what can you do? There are really only 2 options and neither are perfect so you'll need to decide what is most important.
First, there is a 45 watt R20 incandescent light bulb. The shape is similar. They only have 5 less lumens than the 50 watt BR19 and that's not each to tell the difference. The R20 is only 3/16" shorter so the size is pretty similar. Biggest difference??? The R20 is a flood light where the BR19 was a spot light.
So, secondly, if you need a spot and aren't as concerned about the bulb being the same or similar, you can look at a halogen bulb. The 50 Watt PAR20 light bulb. The PAR20 bulbs are going to be 3/4" shorter than the BR19. The beam of spot is the same. 25 degree. The bulbs are very different. The halogen is a heavy bumpy glass to diffuse the lighting. Where the BR19 was a smoother lighter glass. The PAR20 is also going to be a brighter bulb. It has 530 lumens where the BR19 only had 385 lumens.
As you can see, neither is a perfect replacement. But they are the best options available.