So you want to replace your 20" with 17" rims? Yes you can do that but you will need new tires as well. You would be looking at around 800 or 900 or so for rims and about another 800 or 1000 or so for tires. FN Wheels make some good rims with different color choices as well which are made to be Toyota spec
17″ FN Wheels ƒ(x) Pro | FN Wheels Store I paid 899 for 4 shipped with lugnuts.
Oh another thing top consider is you may want 5 tires and rims to account for your spare. I know FN wheels will sell you a cheaper "dinged" wheel that may just have a scratch on it for cheaper.