There are lot of neutron sources:
Not all of them is useful for large scale transmutation since their neutron flux is many orders of magnitude smaller than that of nuclear reactors. However, spallation neutron sources have higher flux than reactors and don't require radioisotopes (They are high energy particle accelerators bombing high-Z materials with protons). Thus, they can transform U-238 into Pu-239, or Thorium-232 (which has half life even exceeding U-238) into U-233.
Since the Thorium fuel cycle has many disadvantages (including especially nastily radioactive daughter elements), and their spallation source will need to have extreme luminosity and, therefore, extreme energy requirements, the production of their first tons of nuclear fuel would require significant effort. (Enriching uranium is, in fact, also quite difficult).
After they have their first critical reactor, they can use it to breed more and more fuel and construct other reactors and nukes.
Thus, if they really want, they can achieve fission before fusion, but they may deem it impractical.
EDIT: It was suggested that without natural fissionables they will never discover fission and have no motive to produce artificial fissionables, but I disagree with this. If their science advances roughly on the same way as ours, the following will happen:
First they discover radioactive decay in long-lived natural isotopes (or in those which are continuously recreated by cosmic radiation) and, thus, get to know that a nucleus is composed of smaller parts. Then they invent cyclotrons and linear accelerators and start to systematically smash together all available isotopes to see what happens. This way they discover the neutron and start to formulate their theories and models about the inner workings of the nucleii. If they are clever enough, they may predict fission purely theoretically and start to search paths to nucleii with the required Z and N in order to test their models.
Even if they can't discover it on paper, one day a diligent scientist will test U-238 or Th-232 against neutron radiation and, thus, discover Plutonium and U-233. The elated researcher will isolate these atoms and test them again against neutrons. He will then find weird mid-sized atoms in the sample and realize that nucleii were split in half. Someone will then notice the energy source potential of the process and formulate a letter to the government...
Edit2: If the civilization builds enough of them, they can produce the needed fuel by spallation sources. But there is a more efficient way to use the spalled neutrons:
Subcritical reactors are safe, and able to convert Th 232 into U 233 effectively. No such reactor was realised so far, but this is a well established design.