Hi Mark.
Tamiya makes an acrylc clear gloss, X-22. You can make this flat or anywhere in between by adding Tamiya X-21 Flat Base.
Note that X-21 is not a flat clear coat, it is an additive only, and must NEVER be applied on its own. Applied on its own (ie. not mixed with paint or clear gloss), it will leave a chalky white frosting on your model.
When mixing X-22 and X-21, a ratio of about 4 parts X-22 (clear) to one part X-21 (Flat Base) will give you a very flat finish. Anywhere between 6-8 parts clear to one part Flat Base will give you a semi-gloss finish. You may have to experiment a little to get the level of sheen you desire.
Out of the bottle, I believe Humbrol makes a semi-gloss clear coat but this is an enamel (or lacquer) product.