Compressive values for wood can be found throughout different wood working databases. However, to understand the compressive value is a bit tricky because it isn't as simple as reading a number value for the wood's compressive abilities. To be honest, I understand you are new to making bows, but you might be thinking about this the wrong way. Your best resource on this information is the experience of the various skilled bowers on this site and other sites because they have done a MASSIVE amount of VERY CAREFUL experimentation between them trying to replicate classic bows and push the designs any type of wood can handle as well as expanding the known database of usable bow woods. Bowers are audacious in this sense. LoefflerChuck has a video on youtube where he goes through the whole process of making an Ishi bow and talks about how he collected his wood. Check that out, he is a great bower. There is a known bowwood database on this site I believe as well as on PaleoPlanet. Although this doesn't usually let you know the compressive values of the wood, it does let you know if a wood is usable for a bow. Past that, what most people do to collect wood is just ask people who have land if they wouldn't mind you searching around for a tree. Usually, if you are willing to give them a bow with any tree you fell ,they will be more than happy to help you out. For Juniper, incense cedar and yew, you only really need a limb. So, although this may not be a popular opinion, I'll just cut a good clean limb off a tree as I see them and call it "pruning". I've never had a problem, but asking is far better. There is no need to cut in a national park. I'd just get some Incense Cedar limbs around your area. As far as sourcing wood....juniper is a bit hard to come by. The most common one you can find is Eastern Red Cedar (which is Juniper and not cedar). You can buy staves of it frequently on eBay or through some retailers on the net. Yew is harder to come by and you don't typically find incense cedar staves as many bowers don't seem to know it is a highly valuable compression wood. I helped a man locate some western Juniper a while back and there is a supplier of western juniper posts in Portland. He stopped by the yard and they helped him find a nice post that was mostly knot and twist free. Only cost him 15$ I think. You want the name of that place? Maybe you can road trip up there. Good luck!