If you're buying a set look up True Blox, I believe they were the first to start making and selling them and they provide a ton of really helpful bodywork info on their website that you can't find anywhere else. The quality of info that they post, and the fact that they take the time to answer questions on their Facebook and Instagram accounts shows how much they care about helping people do the best bodywork possible, they're not just out there to sell blocks. They were only selling small wet sanding sets when I first found out about them so I ordered those, could quickly see how much better acrylic was, so I made my own bigger blocks before they started offering them.
If want to make your own, check to see if a local glass shop has scraps they'll let you have or sell for cheap. We used to be a full service glass shop so we had a bunch of drops that were good sizes to make my blocks from. You'll want 1/8, 3/16, 1/4, and 3/8 in a few different lengths for your main set. 1/16" and thinner works well for smaller blocks. I also like half width blocks in 1/8, 3/16, and 1/4, they're more flexible since they're half the width. The length and width will change how much arc you can get out of them, not just the thickness. Once you make and use a few you start to get the feel of which block you'll need for matching the curve of the panel you're working. I use a couple layers of Gorilla Tape in a T shape for handles, and on some of the blocks I've glued soft 1/4" foam to the back of the block so they're more comfortable. The tape holds up pretty well but will have to be replaced occasionally. They're not the most comfortable to use but they perform the best by far, its an acceptable trade off for the finish they produce. If you're making your own it's important to have straight and parallel sides; if the width is uneven they won't flex in a consistent arc across their length. I wouldn't use a durablock as a handle, at least not on the thinner/more flexible blocks. You'll want a soft handle that doesn't influence how the block flexes- durablocks are too stiff for that.
Even if you buy as set you'll want to keep some acrylic around for making custom blocks to fit the job you're on. These are some of my smaller detail blocks I used on a F100 firewall.
This is the kind of result you can expect using acrylic. The hard surface is super effective at cutting down high spots and texture. This is primer blocked from 80 to 400 with wax and grease remover wiped on to check the reflection. Even moving around at very steep angles there is zero texture and almost no ripples down the 6' bed side.