How can I raise my handlebars when there are no spacers above the stem?

You can buy a handlebar riser adaptor like this:

Replace the stem with one that has a bit of an angle.

Stems are available in many combinations of lengths and angles, so if you're dissatisfied with the reach on the bike too you can solve that in one go as well

Every respectable LBS should have a ton of options on hand.

Btw many forks require a minimum of one spacer above the stem, especially those with carbon steerer tubes. You should double check your stem is installed complaint with the manufacturer's requirements.

You can get another handlebar with a different value of "rise":

There are models with very high rise:

You can go to the extreme (might be very flexy, but OK for e.g. commuting):

A web image search for "stem riser adapter" will locate literally hundreds of sites selling handlebar height increasing adapters - from all the usual large sellers plus numerous smaller outlets.

The image at the bottom of this answer provides a random sampling of examples.
(Click image for larger higher resolution version).

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Here is a useful Stack Exchange Q&A re the safety of using such extenders.

Here are a number of SE questions re handlebar extension.

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(Click image for larger higher resolution version).