The best (free) bottles I have found are those from veterinary or medical clinics, that the sterile water comes in. Ask if you can set up a recycle bin, get all you want. 1 liter and 250ml sizes. Strangely enough, the sterile water has an expiration date, so they usually dump perfectly good water before trashing the bottles. Totally unnecessary.
Ask also about an irrigation syringe, as you will need one to meter your Rodinal.