We used to make our own just by cutting a plasmid with some enzymes. I can’t remember which plasmid we used or which enzymes. Just decide what range of sizes you want, then sit down with a programme that can show you fragment sizes, and find a good combination of enzymes/plasmid that will give you what you want.
Lambda phage DNA digested with HindIII is what everybody used to use. I think maybe a PstI digest of lambda DNA is also a possibility. That means you have to prepare or buy some lambda DNA and one or more restriction enzymes, but if you can afford to buy the DNA and enzymes, you are advised to purchase some ready-made (or ready-cut) marker DNAs. Much simpler and probably more reliable. If you only need sizes up to 3-4 kbp, there is probably of digest of pBR322 that would work, though without looking at a map I don’t know what enzyme to suggest.