The thing about the making of cover or banner thumbnail is it wasn't draw with that small pixel. I will usually use 2000+ pixel sizes to draw then crop and resize it to a smaller thumbnail to fit wherever place or needed to be placed.
Having something drawn that big is that you can freely but and resize it however you want, do know that you wouldt want it to be drawn on 100 something pixel size as the original picture since you wouldn't be able to resize it if you needed it to be bigger in the future. So like one picture, fit everything as long as you resize them properly.
Some peoples in this forum might offer you free drawing if you asked politely but making one drawing took time and efforts, so most preferred to be paid.^^;
And not all commission cost $50+. Just check on the commision threads, there might be some commission artist that will draw you for lower prices.
If you don't want to pay for commission, you can just make your own thumbnail by taking a Cc pictures on the internet and resize or crop it. (Best if you give the owner credit)
Edit. Most commission artist will made the size as ordered, so it is something common. Just ask the artist about it.^^