The answer to your question will depend on what you got and how big of a hurry you're in.
As mentioned, using the tractor hydraulics should get you enough force to split wood; it's the cycle time that's going to be the issue. If its a 15 ton splitter with a short (foot and a half give or take) it will be slow, but possibly manageable. 6-8 gpm would be better, but if it were me, I'd buy whatever your PTO can pull if it's reasonably priced. I don't see the downside of a faster cycle time here, at least within reason. If you've got a 20-25 ton splitter with a stroke much over 20", you'll want 10gpm give or take, and probably 15ish with a 37 ton splitter. With 13 PTO hp, you're probably not good for much more than that, probably a little shy of 20gpm.
This is assuming that the splitter is set up for use with tractor hydraulics and didn't come with a pump. If it had an engine and it blew, seized or whatever, I'd replace it with a $100 HF engine, or a power washer take off from craislist from someone who wrecked their pump. If it's a smaller unit and you're going to be running if frequently, I might still buy and engine and pump even if it is set up for tractor hydraulics. The idea is to put the hours on a $30-100 small engine vs a tractor worth thousands. If it's not going to see much use, I'd go tractor hydraulics or PTO pump. Carbs gunk up if they sit too long and you won't be putting many hours on the tractor anyway. On a really big splitter, I'd probably go lean more towards the PTO pump as the small engine will need to be bigger and more expensive, and they do suck gas.