You can achieve an auto complete if you publish your form via SharePoint on a web page and add a content editor pointing to a little jQuery script as described here:
In that case you don't even have to load in the data via your form first.
In regards to the Suppliers list, I'd really recommend doing that in a different way. Let's say your users add forms which include a lookup field on a supplier - just add another lookup directly in your supplier list back to the lookup field in your form library. What you'll get is a counter and that is really useful. You can now for instance use that column to order your query results by the counter column basically resulting in the suppliers used the most turning up at the top of your dropdown.
If you aren't looking at hundreds or thousands of suppliers you could maybe provide a better user experience if you offered your users buttons to select suppliers instead of asking them to input anything.
You could for example create buttons for each letter of the alphabet and then simply filter a repeating table showing the results of your query by the letter pressed. Each button gets a rule that sends its letter to a hidden text field. When the button is pressed you run a rule that filters the suppliers list by 'Does not begin with' on the text field. If you also add a rule that hides the section in which you show your repeating table if there is no value in your text field, then you won't unnecessarily present your users with a huge list to start with.
Start with the A button and then simply copy it to create the others.
Ok, but how do you then let them pick a supplier? That's the nice and easy part. In your repeating list with the query results of your suppliers list, remove the title box by deleting it. Add a button into the same table field. Right click the button and in the properties change its name to the Title field (click FX). Finally set a rule on the button to set your actual value field - the one you want to submit - to the ID of the returned row. If your supplier column is a lookup to your Supplier list, it'll work fine like that.
Sorry for the long post. Hope it helps a little.
Let me know if you'd like a quick video to explain that a bit more. I can always put one together quickly.