249 Ohms is a standard E-96 value. The next higher E-96 value is 255 Ohms.
The next higher series is the E-196 series. In it the close values are 249 and 252 ohms.
250 Ohms is not a standard value in either series. That doesn't mean that it's not available, just that you'll probably have to have it made special and be willing to pay through the nose. And probably have to buy many tens of thousands of parts.
As for putting a 1 ohm in series with a 249 ohm, remember the effect of component tolerance. A 1% 249 ohm resister could actually have a value as high as 251.49 ohms. Also, each resisters in the system adds it's own error due tho the same tolerance effect. And it's not only resisters that can add errors. Voltage and current references, A2D accuracy, operating temperature, etc. will also affect the precision of the final system.
So, if you really need a tight tolerance through the system, you're better off adding some sort of trimming to the system, and calibrate it with some precision measurement equipment.
Of course, for a homework assignment, you can probably get by with using any calculated value you want to put down, even if you cant actually buy the part.