You can order from Graco, but depending on how many parts you need, you may be better off just buying a new seat. An older one (20 lb limit) may have limited parts available and you might not be able to get matching patterns/colors. Look under customer service on their website www.gracobaby.com - you will need the model number and date of manufacture to find the available parts. You should call to confirm if you want matching patterns, etc. as it won't tell you on the website.
Just for comparison when you look it up, you can get a new front adjust Snugride for $80 and a SafeSeat for $100.
Mama to three boys ('03, '05, '07)