TR-PAK IIa w CL-PAK IIa Battery Cluster for replacement - the original NiCad Cells are shot! They will get very hot if charged and may explode. Ask me how I know... TR-PAK tested good, the CL-PAK must be rebuilt or replaced. Charger condition unknown, it did get hot when I tried it with these batteries.
As Bombon notes, the connector is VITAL to install a new battery, I do not know off any US source. NiCad C-cellls are readily available but relatively low capacity and tend to age quickly if not kept charged. Their one virtue is fast recycling.
I replaced the batteries in mine with a 6-AA NiMH cluster because they were readily available and I had a charger. There are plenty of alternatives. The AAs produced over 100 full power shots on a Sunpak 622, though they did get quite warm. I intend to use mine with a 120J which shouldn't be as demanding.