I already buy Monero using cash-by-mail and gift cards. I am pretty sure I will still be able to do this even if all exchanges shut down. What we need to be doing to prepare is building a parallel crypto-economy wherein crypto is exchanged directly for goods, services, and physical stores if value (cash, gold, silver, platinum, diamond, etc) without ever directly interfacing with banks or the above-ground economy. Monero and Zcash can be mined without suspicion-raising amounts of electricity, so you could easily keep a crypto economy afloat even if mining or possessing it became illegal. I mean, there are already plenty of crypto users currently operating out of countries where private ownership is illegal already.
Not to mention, that if crypto is still legal in just one country, Americans could still purchase legally mined crypto abroad using gift cards, mailable goods, or services.
Even if you are imprisoned, you could potentially still access and use crypto wallets using smuggled cellphones (which prison authorities seem totally unable to eradicate).