Download any genome you want from any source, then subset that file based on the chromosome. Most are distributed as FASTA sequences. You could select among the chromosomes easily (and within a loop!) by using Biostrings in R, amongst a ton of other programming options.

library(Biostrings)
a <- readDNAStringSet("your_genome.fa", format="fasta")

#show the data structure
a

#the chromosome names are stored in
names(a)

#and you could write to a new file with:
b <- a[whatever chromosome you want]

writeXStringSet(b, file="your_chromosome.fa", format="fasta")

See the answer to this question:

A: All completed genomes for a phyla