Can nonmembers in RuneScape get skillcapes?

Yes you can buy skillscapes, but you don't have to. But keep in mind that you also need to be level 99 in that skill to be able to buy the cape. To buy from some NPC, find the NPC wearing that respective skillcape, which is usually a Guild Master, and buy it for 1k. It used to be that you had to be a member to buy skillcapes but now you can buy them if you're a non-member. However, you cannot get some skillcapes if you're a non-member because some of the guilds are located in Members-Only Areas. But if you have friends that are members and have the skill, you might be able to get it from them by asking them to help you buy it.