By sidelock you mean liner lock? If so, some recommendations from my limited folder making experience:
Yes, titanium. The most common you'll find for sale is 6Al4V, which is a good choice for folder springs. Some of the other TI alloys can have less desirable working characteristics. If you don't have a supplier, I suggest Alpha Knife Supply for titanium. Good selection, customer service, and prices.
As for thickness, I'd recommend at least .040". If making a standard type liner lock I like .060-ish TI for the locking liner.
6Al4V is good as is for a spring. TI can be heat treated a little harder but it's not a common or necessary process. Guys who get farther into folders sometimes use a carbidizer to further harden the face of the lockbar, but there are plenty of great folders made without this added feature.
I have a folder WIP on the first page here, it's a framelock but much of the processes are similar to a liner lock. I'm not saying it's the best knife ever made or even the best tutorial, but it's there and may help. Ray Rogers has a good liner lock design tutorial at his website, it goes further into actually designing the lock than I do in my WIP. There are other good threads around too if you search- there is a great one on a framelock build over at Knife Network IIRC. I forget who the maker is right now.
When cutting TI, it's best to use a fine toothed bimetal blade like a Lenox Diemaster II, and cut slow and patient. Break your bandsaw blade in right first if it's new. This will save you money. You can use a cheap band but will probably trash it on this one knife if you do. When grinding TI I always use a respirator, the sparks and fumes generated by grinding it are a bit more freaky than steel.
