Generally, if you want to invest in cryptocurrency index funds through your retirement plan or brokerage account, you will not have enough options. Hence, you need to look for independent index funds that are publicly traded. Basically, the only prerequisite here is that you need to have a brokerage account.
On the other hand, there are also limited traditional index funds for tracking cryptocurrencies. However, there is an alternative for advanced crypto traders.
For instance, there are index fund tokens. Actually, these are cryptocurrencies that act as index funds since they individually track a group of cryptocurrencies. Some examples of such crypto index fund tokens are CRYPTO20, DeFi Pulse Index, NFT Index, and more.
Basically, these tokens are small in size. Hence, they are hard to find if you compare them to major cryptocurrencies. Here, investors buy one big cryptocurrency on a crypto exchange and transfer it to a blockchain wallet.
After that, investors go to a decentralized exchange which consists of a big selection of tokens. Moreover, they trade the larger crypto for a crypto index fund token. However, since this process is complex and not beginner-friendly, it is not recommended.
Rather, try to go for independent and individual cryptocurrency index funds. For instance, a popular cryptocurrency index fund is the Bitwise 10 Crypto Index Fund.